Infectious clone of human parvovirus B19 and methods

ABSTRACT

The invention relates to infectious clones of parvovirus B19, methods of cloning infectious B19 clones, and methods of cloning viral genomes that have secondary DNA structures that are unstable in bacterial cells. A B19 infectious clone and methods of producing B19 infectious clones are useful for producing infectious virus. Infectious virus is useful for identifying and developing therapeutically effective compositions for treatment and/or prevention of human parvovirus B19 infections.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. 10/887,770, filed Jul. 9, 2004, now U.S. Pat. No. 7,598,071, the contents of which is incorporated herein by reference in its entirety.

STATEMENT OF RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

Part of the work performed during the development of this invention utilized United States government funds under the Division of Intramural Research, NHLBI.NIH.

BACKGROUND OF THE INVENTION

Human parvovirus B19 is the only member of the Parvoviridae family known to cause diseases in humans. Parvovirus B19 infection causes fifth disease in children, polyarthropathy syndromes in adults, transient aplastic crisis in patients with underlying chronic hemolytic anemia, and chronic anemia due to persistent infection in immunocompromised patients. Hydrops fetalis and fetal death have been reported after maternal infection with parvovirus B19 during pregnancy (Brown et al., 1994, Crit. Rev. Oncol./Hematol. 16:1-13).

Parvovirus B19 exhibits a selective tropism for erythroid progenitor cells. The virus can be cultured in erythroid progenitor cells from bone marrow, fetal liver cells, and cell lines such as UT7/Epo or KU812Ep6. (Ozawa et al., 1986, Science 233:883-886; Brown et al., 1991, J. Gen. Vir. 72:741-745; Komatsu et al., 1993, Blood 82:456-464; Shimomura et al., 1992, Blood 79:18-24; Miyagawa et al., 1999, J. Virol. Methods 83:45-54). Although the virus can be cultured in these cells very little virus is produced. The selective tropism of the virus is mediated in part by neutral glycolipid globoside (blood group P antigen), which is present on cells of the erythroid lineage (Brown et al., 1993, Science, 262:114-117). The presence of globoside on the surface of a cell is a determinant of viral tropism. Parvovirus B19 has a cytotoxic effect on erythroid progenitor cells in bone barrow and causes interruption of erythrocyte production. Human bone marrow cells that lack globoside on the cell surface are resistant to parvovirus B19 infection (Brown et al., 1994, N. Engl. J. Med., 33:1192-1196).

The ends of the parvovirus B19 genome have long inverted repeats (ITR), which are imperfect palindromes that form double-stranded hairpins. The role of the ITRs in the parvovirus B19 viral life cycle is unknown due to the inability to produce an infectious clone containing complete ITR sequences. In other parvoviruses, ITRs play an important role in the viral life cycle: they serve as primers for the synthesis of the complementary strand of viral DNA and are essential for the replication, transcription, and packaging of virus DNA (Berns, K (1990) in Virology, eds. Fields et al. Raven Press Ltd, NY, pp 1743-1763). Previous attempts to produce an infectious clone of parvovirus B19 were unsuccessful due to deletions in the ITR sequences and the instability of the ITRs in bacterial cells (Deiss et al., 1990, Virology 175:247-254; Shade et al., 1986, J. Virol. 58:921-936). Methods of consistently producing infectious B19 parvovirus in cell culture are not known.

Thus, there remains a need to develop an infectious clone of parvovirus B19. A B19 infectious clone and methods of producing B19 infectious clones can be useful for producing infectious virus. Infectious virus is useful for identifying and developing therapeutically effective compositions for treatment and/or prevention of human parvovirus B19 infections, such as for example, antibodies, attenuated vaccines, and chimeric viral capsid proteins comprising antigenic epitopes.

SUMMARY OF THE INVENTION

One aspect of the invention is directed to methods of cloning a parvovirus B19 viral genome. Clones of viral genomes produced by the methods of the invention are useful for consistently producing infectious virus. Infectious virus is useful for identifying and developing therapeutically effective compositions for treatment and/or prevention of human parvovirus B19 infections, such as for example, antibodies, attenuated vaccines, and chimeric viral capsid proteins comprising antigenic epitopes.

The methods of cloning a parvovirus B19 viral genome generally employ introducing a vector comprising all or a portion of a parvovirus B19 genome into a prokaryotic cell that is deficient in at least one recombinase enzyme; incubating the cells at about 25° C. to 35° C.; and recovering the vector from the prokaryotic cells. An inverted terminal repeat (ITR) may be at the 5′ end or 3′ end or both of the viral genome. In an embodiment, the ITR comprises a nucleic acid sequence of SEQ ID NO:1 or SEQ ID NO:2. In addition to at least one ITR, the viral genome may comprise a nucleic acid sequence encoding at least one or all of VP2, nonstructural protein (NS), or 11-kDa protein. The bacterial cell may be recA1, endA, recB, and/or recJ deficient. In an embodiment, the bacterial cell comprises a genotype of e14-(McrA-)Δ(mcrCB-hsdSMR-mrr)171 endA1 supE44 thi-1 gyrA96 relA1 lac recB recJ sbcC umuC::Tn5 (Kanr) uvrC [F′ proAB lacIqZ.M15 Tn10 (Tetr)]. Vectors that are useful in the methods of the invention include pBR322, p ProExHTb, pUc19 and pBluescript SK.

In some embodiments, the full length B19 genome is cloned by cloning at least two portions of the viral genome into separate vectors and recombining the two portions into a single vector. Preferably, two portions of the viral genome comprise an ITR at the end of the portion. The portions of the viral genome can be obtained by digesting the genome with a restriction enzyme that cuts the genome at a location between the ITRs. Preferably the restriction enzyme cuts the genome at a location at least about 800 nucleotides from the ITR. The portions may be cut and religated to reduce the vector size and eliminate undesired restriction sites. For example, the B19 genome may be digested with BamHI. The two fragments (right end genome fragment and left end genome fragment) generated by BamHI digestion are ligated into separate vectors and the full-length genome is generated by recombining the right end genome fragment and left end genome fragment into a single vector. In an embodiment, the full length genome comprises a nucleic acid sequence of SEQ ID NO:5.

The methods of producing or identifying an infectious clone or infectious virus of parvovirus B19 generally employ introducing a vector comprising all or a portion of a viral genome of parvovirus B19 into a population of cells, wherein the vector is present in at least about 15% of the cells; and incubating the cells under conditions to allow for viral replication. Preferably, the cells are eukaryotic cells, more preferably permissive cells such as for example erythroid progenitor cells, fetal liver cells, UT7/EPO cells, UT7/EPO-S1 cells, or KU812Ep6 cells. In some embodiments, the cells are cultured in vitro. Optionally, viral replication can be detected in the cells.

The vector may be introduced into the cells using standard transfection techniques known in the art. In an embodiment, the cells are transfected by electroporation or electrical nuclear transport. The viral genome preferably comprises an ITR sequence having a nucleic acid sequence of SEQ ID NO:1 or SEQ ID NO:2 and a nucleic acid sequence encoding one or more of 11-kDa protein, NS protein, VP1, VP2, or putative protein X. Preferably the ITRs are located at the 5′ end or 3′ end of the genome. In an embodiment, the infectious clone comprises a polynucleotide nucleic acid sequence of SEQ ID NO:5. Reproduction of the infectious clones produced by the methods of the invention can be detected by contacting permissive cells with supernatant from the population of cells and analyzing the contacted cells for spliced capsid transcripts or capsid proteins.

Another aspect of the invention is directed to isolated infectious parvovirus B19 clones. The clones may be produced by the methods of the invention. Infectious B19 clones are useful in diagnostic assays, identifying and developing therapeutically effective compositions for treatment and/or prevention of human parvovirus B19 infections, such as for example, antibodies, attenuated vaccines, and chimeric viral capsid proteins comprising antigenic epitopes. Preferably the clones comprise all or a portion of a parvovirus B19 genome and a replicable vector. The genome may comprise ITRs located at the 5′ end and 3′ end of the genome and a nucleic acid sequence encoding one or more of VP1, VP2, NS, 11-kDa protein, 7.5-kDa protein, or putative protein X. Preferably the viral genome comprises a polynucleotide having at least 90% nucleic acid sequence identity to SEQ ID NO:5 or SEQ ID NO:24. In an embodiment, the viral genome comprises a nucleotide sequence of SEQ ID NO:5.

The invention also encompasses using the infectious parvovirus B19 clone of the invention and/or host cells comprising the clone as immunogenic compositions to prepare vaccine components and/or to develop antibodies that can be used in diagnostic assays or to inhibit or antagonize B19 infection of cells. Host cell cultures comprising the parvovirus B19 clone can be heat inactivated and used as an immunogen. Passaging of an infectious clone in vitro can provide an attenuated strain of parvovirus B19 useful in vaccine compositions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a map of recombinant plasmid pB19-N8, which contains an insert comprising 4844 nucleotides of the B19 genome. The arrows indicate genes. Shaded arrows indicate genes in the B19 genome. The shaded circles at the 5′ and 3′ ends of the B19 genome indicate the ITR sequences.

FIG. 2A shows the structure of the B19 terminal inverted repeats in hairpin form. The “flip” (SEQ ID NO:1) and “flop” (SEQ ID NO:2) orientations at the 5′ end (+ strand) are shown.

FIG. 2B shows a comparison of nucleic acid sequences encoding the flip and flop forms of ITRs in B19 isolate J35 (SEQ ID NOS: 1 and 2), the B19 isolate reported by Deiss et al., 1990, Virology, 175:247-254 (SEQ ID NOS:3 and 4), and the flop form in B19-Hv (SEQ ID NO:37). Aligned positions are boxed in black. The numbering indicates the positions of nucleotides in the genomes of the respective B19 isolates.

FIG. 3 shows the experimental strategy used to construct an infectious clone of parvovirus B19.

FIG. 4 shows a map of recombinant plasmid pB19-4244, which contains an insert comprising 5592 nucleotides of full-length B19 genome (SEQ ID NO:5). The arrows indicate genes. Shaded arrows indicate genes in the B19 genome. The shaded circles at the 5′ and 3′ ends of the B19 genome indicate the ITR sequences.

FIG. 5 shows a map of recombinant plasmid pB19-4244d. The arrows indicate genes. The shaded circles at the 5′ and 3′ ends of the B19 genome indicate the ITR sequences. Shaded arrows indicate genes in the B19 genome. Plasmid pB19-4244d was modified from pB19-4244 by XhoI-Ecl36II digestion to remove the undesired XbaI site.

FIG. 6 shows a map of recombinant plasmid pB19-M20. The arrows indicate genes. Shaded arrows indicate genes in the B19 genome. The shaded circles at the 5′ and 3′ ends of the B19 genome indicate the ITR sequences. Nucleic acid residue 2285 was substituted (C2285T) generating a DdeI site in the B19 genome.

FIG. 7 shows a schematic representation of the replication of B19 viral genome. The replicative DNA form provides evidence of viral DNA replication and can be distinguished by BamHI restriction enzyme digestion.

FIG. 8 shows RT-PCR analysis of UT7/Epo-S1 cells for B19 transcripts. The cells were transfected with recombinant plasmids or infected with B19 virus. Total RNA was extracted from the cells 72 h post-transfection or 72 h post-infection. RT-PCR was performed with a primer pair of B19-1 (SEQ ID NO:6) and B19-9 (SEQ ID NO:7). The PCR products were separated by agarose electrophoresis and analyzed by Southern blotting with an alkaline-phosphatase-labeled probe. (+) and (−) indicate the presence or absence respectively of reverse transcriptase in the PCR reaction. The numbers with arrows indicate amplicon size in base pairs (bp).

FIGS. 9A-C show detection of B19 capsid proteins in UT7/Epo-S1 cells infected with B19 virus (FIG. 9A), UT7/Epo-S1 cells transfected with pB19-M20 (FIG. 9B), and UT7/Epo-S1 cells transfected with pB19-N8 (FIG. 9C). The B19 capsid proteins were detected 72 h post-transfection or 72 h post-infection using monoclonal antibody 521-5D (gift from Dr. Larry Anderson, Centers for Disease Control and Prevention, Atlanta, Ga.). Magnification is 750×.

FIG. 10 shows Southern blot analysis of DNA purified from cells transfected with SalI digested fragment of pB19-M20 or pB19-4244. DNA from B19 virus was used as a positive control. The purified DNA was digested with BamHI or EcoRI and the fragments separated by agarose electrophoresis. The fragments were probed with a ³²P-random-primed probed of the complete B19 coding region. Distinct doublets of 1.5 and 1.4 kb were detected in transfected cell samples digested with BamHI, but not in the plasmid controls. The 1.4 kb band is a definitive marker for viral genome replication.

FIG. 11 shows Southern blot analysis of DNA purified from cells transfected with undigested pB19-M20 or pB19-4244. The purified DNA was digested with BamHI or EcoRI and the fragments separated by agarose electrophoresis. The fragments were detected with a ³²P-random-primed probe of the complete B19 coding region. Distinct doublets of 1.5 and 1.4 kb were detected in transfected cell samples digested with BamHI. The 1.4 kb band is a definitive marker for viral genome replication. A band with a molecular size of 5.6 kb, which corresponds to the size of the B19 genome, was also detected in EcoRI digested DNA. The 5.6 kb band indicated that progeny viral DNA was produced by the transfected cells as neither the B19 genome nor the vector contained an EcoRI restriction enzyme site.

FIGS. 12A and 12B show RT-PCR analysis of UT7/Epo-S1 cells infected with clarified supernatant from B19-infected or p19-4244, pB19-M20, or pB19-N8 transfected cells for B19 transcripts. Total RNA was extracted from the cells 0 h (FIG. 12A) and 72 h (FIG. 12B) post-infection. RT-PCR was performed with a primer pair of B19-1 (SEQ ID NO:6) and B19-9 (SEQ ID NO:7). The PCR products were separated by agarose electrophoresis and analyzed by Southern blotting with an alkaline-phosphatase-labeled probe. (+) and (−) indicate the presence or absence respectively of reverse transcriptase in the PCR reaction. The numbers with arrows indicate amplicon size in base pairs (bp).

FIGS. 13A-C show detection of B19 capsid proteins in cells infected with clarified supernatant from B19-infected (FIG. 13A), or pB19-M20 (FIG. 13B) or pB19-N8 (FIG. 13C) transfected cells. B19 capsid proteins were detected 72 h post-infection using monoclonal antibody 521-5D. Magnification is 750×.

FIG. 14 shows a comparison of a portion of nucleic acid sequence from B19 clone J35 and B19 clone M20. M20 virus has a Ddel restriction site that is not present in J35 virus. FIG. 14 discloses SEQ ID NOS 39 and 40, respectively, in order of appearance.

FIG. 15 shows RT-PCR analysis of B19 transcripts in UT7/Epo-S1 cells infected with J35 virus or infectious clone pB19-M20. cDNA derived from the infected cells was amplified using a primer pair of B19-2255 (SEQ ID NO:8) and B19-2543 (SEQ ID NO:9). The PCR products were digested with DdeI and analyzed by gel electrophoresis. (+) and (−) indicate the presence or absence respectively of reverse transcriptase in the PCR reaction. The numbers with arrows indicate amplicon size in base pairs (bp).

FIG. 16A-F shows RT-PCR analysis of B19 transcripts in UT7/Epo-S1 cells transfected with pB19-M20 (FIG. 16A), pB19-M20/NS (FIG. 16A), pB19-M20/VP1(−) (FIG. 16B), pB19-M20/11(−) (FIG. 16C), pB19-M20/7.5(−) (FIG. 16D), pB19-M20/X(−) (FIG. 16E), or pB19-N8 (FIG. 16F). At 72 h post-transfection, cells were infected with clarified supernatant from the transfected cells. Total RNA was extracted from the cells 72 h post-tranfection or 72 h post-infection. RT-PCR was performed with a primer pair of B19-1 (SEQ ID NO:6) and B19-9 (SEQ ID NO:7). The PCR products were separated by gel electrophoresis. (+) and (−) indicate the presence or absence respectively of reverse transcriptase in the PCR reaction.

FIGS. 17A-D show detection of B19 capsid proteins and 11-kDa protein in cells transfected with pB19-M20 (FIGS. 17A and 17B respectively) or pB19-M20/11(−) (FIGS. 17C and 17D respectively). B19 capsid proteins were detected 72 h post-infection using monoclonal antibody 521-5D (FIG. 17A; 17C). 11-kDa protein was detected 72 h post-transfection using a rabbit polyclonal anti-11-kDa protein antibody (FIGS. 17B, 17D).

DETAILED DESCRIPTION OF THE INVENTION I. Definitions

The term “antibody” is used in the broadest sense and specifically includes, for example, single anti-parvovirus B19 monoclonal antibodies, anti-parvovirus B19 antibody compositions with polyepitopic specificity, single chain anti-parvovirus B19 antibodies, and fragments of anti-parvovirus B19 antibodies. The term “monoclonal antibody” as used herein refers to an antibody obtained from a population of substantially homogeneous antibodies, i.e., the individual antibodies comprising the population are identical except for possible naturally-occurring mutations that may be present in minor amounts.

“Antibody fragments” comprise a portion of an intact antibody, preferably the antigen binding or variable region of the intact antibody. Examples of antibody fragments include Fab, Fab′, F(ab′)₂, and Fv fragments; diabodies; linear antibodies (Zapata et al., 1995, Protein Eng., 8:1057-1062); single-chain antibody molecules; and multispecific antibodies formed from antibody fragments.

The term “binds specifically” refers to an antibody that binds parvovirus B19 and does not substantially bind other parvoviruses. In some embodiments, the antibody specifically binds a first B19 isolate and does not bind a second B19 isolate. For example, an antibody may specifically bind B19-Au and not bind B19-HV.

“Carriers” as used herein include pharmaceutically acceptable carriers, excipients, or stabilizers, which are nontoxic to the cell or mammal being exposed thereto at the dosages and concentrations, employed. Often the physiologically acceptable carrier is an aqueous pH buffered solution. Examples of physiologically acceptable carriers include buffers such as phosphate, citrate, and other organic acids; antioxidants including ascorbic acid; low molecular weight (less than about 10 residues) polypeptide; proteins, such as serum albumin, gelatin, or immunoglobulins; hydrophilic polymers such as polyvinylpyrrolidone; amino acids such as glycine, glutamine, asparagine, arginine or lysine; monosaccharides, disaccharides, and other carbohydrates including glucose, mannose, or dextrins; chelating agents such as EDTA; sugar alcohols such as mannitol or sorbitol; salt-forming counterions such as sodium; and/or nonionic surfactants such as TWEEN™, polyethylene glycol (PEG), and PLURONICS™.

The term “parvovirus B19”, “B19”, “B19 virus”, “B19 clone”, or “B19 isolate” means an isolate, clone or variant B19 viral genome of parvovirus B19 of the family Parvoviridae including genotypes 1, 2, and 3. A naturally occurring isolate of parvovirus B19 of the invention has at least 90% nucleic acid identity to human parvovirus B19-Au (GenBank accession number M13178; SEQ ID NO:24), which lacks intact ITRs at both 5′ and 3′ ends of the genome (Shade et al., 1986, J. Virol., 58:921-936). B19 has a non-enveloped, icosahedral capsid packaging a single-stranded DNA genome of approximately 5600 nucleotides. Transcription of the B19 genome is controlled by the single promoter p6 located at map unit 6, which regulates the synthesis of viral proteins including, but not limited to, nonstructural protein (NS), capsid proteins VP1 and VP2, 11-kDa protein, 7.5-kDa protein, and putative protein X. B19 viral DNA can be isolated from infected humans or cells or can be prepared as described herein. An embodiment of an isolate of parvovirus B19 has a nucleotide sequence of SEQ ID NO:5 (Table 1). In some embodiments, the B19 genome cloned into the vector may have from 1 to about 5 nucleotides deleted from the 5′ end and/or 3′ end of the full length viral genome. For example, the B19 genome (SEQ ID NO:5) cloned into pB19-4244 (FIG. 4) has 2 nucleic acids deleted from the 5′ end and 3′ end compared to the nucleic acid sequence of the full length genome (SEQ ID NO:38).

TABLE 1    1   aaatcaga tgccgccggt cgccgccggt aggcgggact tccggtacaa gatggcggac   59 aattacgtca tttcctgtga cgtcatttcc tgtgacgtca cttccggtgg gcgggacttc  119 cggaattagg gttggctctg ggccagcttg cttggggttg ccttgacact aagacaagcg  179 gcgcgccgct tgatcttagt ggcacgtcaa ccccaagcgc tggcccagag ccaaccctaa  239 ttccggaagt cccgcccacc ggaagtgacg tcacaggaaa tgacgtcaca ggaaatgacg  299 taattgtccg ccatcttgta ccggaagtcc cgcctaccgg cggcgaccgg cggcatctga  359 tttggtgtct tcttttaaat tttagcgggc ttttttcccg ccttatgcaa atgggcagcc  419 attttaagtg ttttactata attttattgg tcagttttgt aacggttaaa atgggcggag  479 cgtaggcggg gactacagta tatatageac agcactgccg cagctctttc tttctgggct  539 gctttttcct ggactttctt gctgtttttt gtgagctaac taacaggtat ttatactact  599 tgttaatata ctaacatgga gctatttaga ggggtgcttc aagtttcttc taatgttctg  659 gactgtgcta acgataactg gtggtgctct ttactagatt tagacacttc tgactgggaa  719 ccactaactc atactaacag actaatggca atatacttaa gcagtgtggc ttctaagctt  779 gaccttaccg gggggccact agcagggtgc ttgtactttt ttcaagcaga atgtaacaaa  839 tttgaagaag gctatcatat tcatgtggtt attggggggc cagggttaaa ccccagaaac  899 ctcacagtgt gtgtagaggg gttatttaat aatgtacttt atcactttgt aactgaaaat  959 gtgaagctaa aatttttgcc aggaatgact acaaaaggca aatactttag agatggagag 1019 cagtttatag aaaactattt aatgaaaaaa atacctttaa atgttgtatg gtgtgttact 1079 aatattgatg gatatataga tacctgtatt tctgctactt ttagaagggg agcttgccat 1139 gccaagaaac cccgcattac cacagccata aatgatacta gtagcgatgc tggggagtct 1199 agcggcacag gggcagaggt tgtgccattt aatgggaagg gaactaaggc tagcataaag 1259 tttcaaacta tggtaaactg gttgtgtgaa aacagagtgt ttacagagga taagtggaaa 1319 ctagttgact ttaaccagta cactttacta agcagtagtc acagtggaag ttttcaaatt 1379 caaagtgcac taaaactagc aatttataaa gcaactaatt tagtgcctac tagcacattt 1439 ttattgcata cagactttga gcaggttatg tgtattaaag acaataaaat tgttaaattg 1499 ttactttgtc aaaactatga ccccctattg gtggggcagc atgtgttaaa gtggattgat 1559 aaaaaatgtg gcaagaaaaa tacactgtgg ttttatgggc cgccaagtac aggaaaaaca 1619 aacttggcaa tggccattgc taaaagtgtt ccagtatatg gcatggttaa ctggaataat 1679 gaaaactttc catttaatga tgtagcagga aaaagcttgg tggtctggga tgaaggtatt 1739 attaagtcta caattgtaga agctgcaaaa gccattttag gcgggcaacc caccagggta 1799 gatcaaaaaa tgcgtggaag tgtagctgtg cctggagtac ctgtggttat aaccagcaat 1859 ggtgacatta cttttgttgt aagcgggaac actacaacaa ctgtacatgc taaagcctta 1919 aaagagcgca tggtaaagtt aaactttact gtaagatgca gccctgacat ggggttacta 1979 acagaggctg atgtacaaca gtggcttaca tggtgtaatg cacaaagctg ggaccactat 2039 gaaaactggg caataaacta cacttttgat ttccctggaa ttaatgcaga tgccctccac 2099 ccagacctcc aaaccacccc aattgtcaca gacaccagta tcagcagcag tggtggtgaa 2159 agctctgaag aactcagtga aagcagcttt tttaacctca tcaccccagg cgcctggaac 2219 actgaaaccc cgcgctctag tacgcccatc cccgggacca gttcaggaga atcatttgtc 2279 ggaagcccag tttcctccga agttgtagct gcatcgtggg aagaagcctt ctacacacct 2339 ttggcagacc agtttcgtga actgttagtt ggggttgatt atgtgtggga cggtgtaagg 2399 ggtttacctg tgtgttgtgt gcaacatatt aacaatagtg ggggaggctt gggactttgt 2459 ccccattgca ttaatgtagg ggcttggtat aatggatgga aatttcgaga atttacccca 2519 gatttggtgc gatgtagctg ccatgtggga gcttctaatc ccttttctgt gctaacctgc 2579 aaaaaatgtg cttacctgtc tggattgcaa agctttgtag attatgagta aagaaagtgg 2639 caaatggtgg gaaagtgatg atgaatttgc taaagctgtg tatcagcaat ttgtggaatt 2699 ttatgaaaag gttactggaa cagacttaga gcttattcaa atattaaaag atcattataa 2759 tatttcttta gataatcccc tagaaaaccc atcctctctg tttgacttag ttgctcgcat 2819 taaaaataac cttaaaaatt ctccagactt atatagtcat cattttcaaa gtcatggaca 2879 gttatctgac cacccccatg ccttatcatc cagtagcagt catgcagaac ctagaggaga 2939 agatgcagta ttatctagtg aagacttaca caagcctggg caagttagcg tacaactacc 2999 cggtactaac tatgttgggc ctggcaatga gctacaagct gggcccccgc aaagtgctgt 3059 tgacagtgct gcaaggattc atgactttag gtatagccaa ctggctaagt tgggaataaa 3119 tccatatact cattggactg tagcagatga agagctttta aaaaatataa aaaatgaaac 3179 tgggtttcaa gcacaagtag taaaagacta ctttacttta aaaggtgcag ctgcccctgt 3239 ggcccatttt caaggaagtt tgccggaagt tcccgcttac aacgcctcag aaaaataccc 3299 aagcatgact tcagttaatt ctgcagaagc cagcactggt gcaggagggg ggggcagtaa 3359 tcctgtcaaa agcatgtgga gtgagggggc cacttttagt gccaactctg tgacttgtac 3419 attttctaga cagtttttaa ttccatatga cccagagcac cattataagg tgttttctcc 3479 cgcagcaagt agctgccaca atgccagtgg aaaggaggca aaggtttgca ccattagtcc 3539 cataatggga tactcaaccc catggagata tttagatttt aatgctttaa acttattttt 3599 ttcaccttta gagtttcagc acttaattga aaattatgga agtatagctc ctgatgcttt 3659 aactgtaacc atatcagaaa ttgctgttaa ggatgttaca gacaaaactg gagggggggt 3719 gcaggttact gacagcacta cagggcgcct atgcatgtta gtagaccatg aatacaagta 3779 cccatatgtg ttagggcaag gtcaagatac tttagcccca gaacttccta tttgggtata 3839 ctttccccct caatatgctt acttaacagt aggagatgtt aacacacaag gaatttctgg 3899 agacagcaaa aaattagcaa gtgaagaatc agcattttat gttttggaac acagttcttt 3959 tcagctttta ggtacaggag gtacagcaac tatgtcttat aagtttcctc cagtgccccc 4019 agaaaattta gagggctgca gtcaacactt ttatgagatg tacaatccct tatacggatc 4079 ccgcttaggg gttcctgaca cattaggagg tgacccaaaa tttagatctt taacacatga 4139 agaccatgca attcagcccc aaaacttcat gccagggcca ctagtaaact cagtgtctac 4199 aaaggaggga gacagctcta atactggagc tgggaaagcc ttaacaggcc ttagcacagg 4259 tacctctcaa aacactagaa tatccttacg cccggggcca gtgtctcagc cgtaccacca 4319 ctgggacaca gataaatatg tcacaggaat aaatgctatt tctcatggtc agaccactta 4379 tggtaacgct gaagacaaag agtatcagca aggagtgggt agatttccaa atgaaaaaga 4439 acagctaaaa cagttacagg gtttaaacat gcacacctac tttcccaata aaggaaccca 4499 gcaatataca gatcaaattg agcgccccct aatggtgggt tctgtatgga acagaagagc 4559 ccttcactat gaaagccagc tgtggagtaa aattccaaat ttagatgaca gttttaaaac 4619 tcagtttgca gccttaggag gatggggttt gcatcagcca cctcctcaaa tatttttaaa 4679 aatattacca caaagtgggc caattggagg tattaaatca atgggaatta ctaccttagt 4739 tcagtatgcc gtgggaatta tgacagtaac catgacattt aaattggggc cccgtaaagc 4799 tacgggacgg tggaatcctc aacctggagt atatcccccg cacgcagcag gtcatttacc 4859 atatgtacta tatgacccta cagctacaga tgcaaaacaa caccacagac atggatatga 4919 aaagcctgaa gaattgtgga cagccaaaag ccgtgtgcac ccattgtaaa cactccccac 4979 cgtgccctca gccaggatgc gtaactaaac gcccaccagt accacccaga ctgtacctgc 5039 cccctcctat acctataaga cagcctaaca caaaagatat agacaatgta gaatttaagt 5099 atttaaccag atatgaacaa catgttatta gaatgttaag attgtgtaat atgtatcaaa 5159 atttagaaaa ataaacgttt gttgtggtta aaaaattatg ttgttgcgct ttaaaaattt 5219 aaaagaagac accaaatcag atgccgccgg tcgccgccgg taggcgggac ttccggtaca 5279 agatggcgga caattacgtc atttcctgtg acgtcatttc ctgtgacgtc acttccggtg 5339 ggcggaactt ccggaattag ggttggctct gggccagcgc ttggggttga cgtgccacta 5399 agatcaagcg gcgcgccgct tgtcttagtg tcaaggcaac cccaagcaag ctggcccaga 5459 gccaacccta attccggaag tcccgcccac cggaagtgac gtcacaggaa atgacgtcac 5519 aggaaatgac gtaattgtcc gccatcttgt accggaagtc ccgcctaccg gcggcgaccg 5579 gcggcatctg attt

“Variants” of the parvovirus B19 viral genome refer to a sequence of a viral genome that differs from a reference sequence and includes “naturally occurring” variants as well as variants that are prepared by alteration of one more nucleotides. In some embodiments, when the viral genome has the sequence of a naturally occurring isolate, the reference sequence may be human parvovirus B19-Au (GeneBank accession number M13178; SEQ ID NO:24), which lacks intact ITRs at both 5′ and 3′ ends of the genome and the variant has at least 90% sequence identity to the reference sequence. In other cases, a variant may be prepared by altering or modifying the nucleic acid sequence of the viral genome including by addition, substitution, and deletion of nucleotides. In that case, the reference sequence can be that of parvovirus B19 comprising a polynucleotide sequence of SEQ ID NO:5. In some embodiments, a parvovirus genome has at least 90% sequence identity, more preferably at least 91%, more preferably at least 92%, more preferably at least 93%, more preferably at least 94%, more preferably at least 95%, more preferably at least 96%, more preferably at least 97%, more preferably at least 98%, more preferably at least 99% or greater sequence identity to that of a parvovirus B19 genome comprising a nucleic acid sequence of parvovirus B19 Au (GeneBank accession number M13178; SEQ ID NO:24) or a parvovirus B19 comprising a polynucleotide sequence of SEQ ID NO:5.

An “infectious clone” of parvovirus B19 as used herein refers to a full-length genome or portion of a genome of a parvovirus B19 isolate cloned into a replicable vector that provides for amplification of the viral genome in a cell. In some embodiments, a portion of the parvovirus B19 genome comprises or consists of nucleic acid sequence encoding at least one ITR, VP2, NS, and 11-kDa in a single replicable vector. In other embodiment, the viral genome is a full-length genome. The replicable vector provides for introduction and amplification of the viral genome in a wide variety of prokaryotic and eukaryotic cells

The term “erythroid progenitor cell” as used herein refers to a red blood cell precursor cell that differentiates to produce red blood cells.

“Electrical nuclear transport” is a method of introducing nucleic acids into cells such as eukaryotic cells using an electrical current. In some embodiments, in electrical nuclear transport, a recombinant plasmid is transported into the nucleus of cells. Greater amounts of DNA are transported into the nucleus of dividing cells with electrical nuclear transport than may be expected by cell division alone, thereby substantially increasing the likelihood of integration of complete expression cassettes. Electrical nuclear transport methods and buffer systems are described in U.S. 20040014220.

The term “full length genome” refers to a complete coding sequence of a viral genome that comprises at least 75% or greater of the nucleotide sequence that forms the hairpin of the ITR at the 5′ end and 3′ end of the genome. In an embodiment, the coding sequence comprises nucleic acid sequence encoding VP1, VP2, NS, 11-kDa protein, 7.5-kDa protein, and putative protein X. In another embodiment, ITRs at each end of the full length clone may have 1 to about 5 deletions at each end and retain the ability to provide for replication and expression of viral proteins. In preferred embodiments, the ITR has at least about 94%, more preferably 95%, more preferable 96%, more preferably 97%, more preferably 98%, more preferably about 99%, and more preferably 100% of the sequence of that of viral genome isolated from nature, such as that of SEQ ID NO:5 or SEQ ID:24.

The terms fusion protein” and a “fusion polypeptide” refer to a polypeptide having two portions covalently linked together, where each of the portions is a polypeptide having a different property. The property may be a biological property, such as activity in vitro or in vivo. The property may also be a simple chemical or physical property, such as binding to a target molecule, catalysis of a reaction, etc. The two portions may be linked directly by a single peptide bond or through a peptide linker containing one or more amino acid residues. Generally, the two portions and the linker will be in reading frame with each other.

The term “infection” as used herein refers to the introduction B19 viral DNA into a cell wherein introduction of the viral DNA into the cell is mediated by B19 capsid. Cells are typically infected by contacting the cell with B19 virus. Infection of a cell by B19 virus may be determined by analyzing the cell for increase in viral DNA including by detecting the presence or increase of spliced capsid transcripts and/or unspliced NS transcripts and/or capsid proteins.

The term “immunogenic effective amount” of a parvovirus B19 or component of a parvovirus refers to an amount of a parvovirus B19 or component thereof that induces an immune response in an animal. The immune response may be determined by measuring a T or B cell response. Typically, the induction of an immune response is determined by the detection of antibodies specific for parvovirus B19 or component thereof.

An “isolated” antibody is an antibody that has been identified and separated and/or recovered from a component of its natural environment. Contaminant components of its natural environment are materials that would interfere with diagnostic or therapeutic uses for the antibody, and may include enzymes, hormones, and other proteinaceous or nonproteinaceous solutes. Isolated antibody includes the antibody in situ within recombinant cells since at least one component of the antibody's natural environment will not be present. Ordinarily, however, isolated antibody will be prepared by at least one purification step.

An “isolated” nucleic acid molecule is a nucleic acid molecule that is identified and separated from at least one contaminant nucleic acid molecule with which it is ordinarily associated in the natural source. Preferably, the isolated nucleic is free of association with all components with which it is naturally associated. An isolated nucleic acid molecule is other than in the form or setting in which it is found in nature. Isolated nucleic acid molecules encoding, for example, B19 genome or B19 viral proteins therefore are distinguished from nucleic acid molecules encoding B19 viral proteins or a B19 genome as it may exist in nature. In an embodiment, the B19 genome comprises a nucleic acid sequence encoding one or more of 11-kDa protein, VP1, VP2, NS, 7.5-kDa protein, and protein X. In another embodiment, the polynucleotides have a nucleotide sequence that encodes a B19 genome that has greater than 99% nucleic acid sequence identity to SEQ ID NO:5 (Table 1). Preferably, the polynucleotide encodes an infectious clone of parvovirus B19.

“ITR” or “ITR sequence” refers to an inverted terminal repeat of nucleotides in a nucleic acid such as a viral genome. The ITRs include an imperfect palindrome that allows for the formation of a double stranded hairpin with some areas of mismatch that form bubbles. The ITRs serve as a primer for viral replication and contain a recognition site for NS protein that may be required for viral replication and assembling. In some embodiments, the location and number of the bubbles or areas of mismatch are conserved as well as the NS binding site. The NS binding site provides for cleavage and replication of the viral genome. In an embodiment, the parvovirus B19 genome comprises one or more ITR sequences. Preferably, the B19 genome comprises an ITR sequence at the 5′ end and the 3′ end. An ITR may be about 350 nucleotides to about 400 nucleotides in length. An imperfect palindrome may be formed by about 350 to about 370 of the distal nucleotides, more preferably about 360 to about 365 of the distal nucleotides. Preferably the imperfect palindrome forms a double-stranded hairpin. In an embodiment, the ITRs are about 383 nucleotides in length, of which about 365 of the distal nucleotides are imperfect palindromes that form double-stranded hairpins. In another embodiment, the ITRs are about 381 nucleotides in length, of which about 361 of the distal nucleotides are imperfect palindromes that form double-stranded hairpins. In some embodiments, a B19 genome comprises at least 75% of the nucleotide sequence that forms the hairpin in the ITR at the 5′ end and 3′ end of the genome. In other embodiments, the ITRs may have 1 to about 5 nucleotides deleted from each end. In a further embodiment, the ITRs comprise a nucleic acid sequence of SEQ ID NO:1 and/or SEQ ID NO:2. The ITRs may be in the “flip” or “flop” orientation.

The term “permissive cells” means cells in which parvovirus B19 isolates can be cultured. The permissive cells are eukaryotic cells. Examples of permissive cells include, but are not limited to primary erythroid progenitor cells from bone marrow, blood, or fetal liver cells, megakaryoblast cells, UT7/Epo cells, UT7/Epo-S1 cells, KU812Ep6 cells, JK-1 cells, and MB-O2 cells.

“Percent (%) nucleic acid sequence identity” with respect to the nucleic acid sequences identified herein is defined as the percentage of nucleotides in a candidate sequence that are identical with the nucleotides in a reference B19 nucleic acid sequence, after aligning the sequences and introducing gaps, if necessary, to achieve the maximum percent sequence identity. In some embodiments, the reference B19 nucleic acid sequence is that of SEQ ID NO:5 or that of SEQ ID NO:24. Alignment for purposes of determining percent nucleic acid sequence identity can be achieved in various ways that are within the skill in the art, for instance, using publicly available computer software such as BLAST, BLAST-2, ALIGN, ALIGN-2 or Megalign (DNASTAR) software. Those skilled in the art can determine appropriate parameters for measuring alignment, including any algorithms needed to achieve maximal alignment over the full-length of the sequences being compared.

For purposes herein, the % nucleic acid sequence identity of a given nucleic acid sequence A to, with, or against a given nucleic acid sequence B (which can alternatively be phrased as a given nucleic acid sequence A that has or comprises a certain % nucleic acid sequence identity to, with, or against a given nucleic acid sequence B) is calculated as follows: 100 times the fraction W/Z where W is the number of nucleotides scored as identical matches by the sequence alignment program in that program's alignment of A and B, and where Z is the total number of nucleotides in B. It will be appreciated that where the length of nucleic acid sequence A is not equal to the length of nucleic acid sequence B, the % nucleic acid sequence identity of A to B will not equal the % nucleic acid sequence identity of B to A.

“Recombinant” refers to a polynucleotide that has been isolated and/or altered by the hand of man or a B19 clone encoded by such a polynucleotide. A DNA sequence encoding all or a portion of a B19 viral genome may be isolated and combined with other control sequences in a vector. The other control sequences may be those that are found in the naturally occurring gene or others. The vector provides for introduction into host cells and amplification of the polynucleotide. The vectors described herein for B19 clones are introduced into cells and cultured under suitable conditions as known to those of skill in the art. Preferably, the host cell is a bacterial cell or a permissive cell.

The term “transformation” as used herein refers to introducing DNA into a bacterial cell so that the DNA is replicable, either as an extrachromosomal element or by chromosomal integrant. Depending on the host cell used, transformation is done using standard techniques appropriate to such cells. The calcium treatment employing calcium chloride is generally used for bacterial cells that contain substantial cell-wall barriers. Another method for transformation is electroporation.

The term “transfection” as used herein refers to introducing DNA into a eukaryotic cell so that the DNA is replicable, either as an extrachromosomal element or by chromosomal integrant. Depending on the host cell used, transfection is done using standard techniques appropriate to such cells. Methods for transfecting eukaryotic cells include polyethyleneglycol/DMSO, liposomes, electroporation, and electrical nuclear transport.

The term “transfection efficiency” as used herein means the percentage of total cells contacted with a nucleic acid, such as a plasmid, that take up one or more copies of the plasmid. Tranfection efficiency can also be expressed as the total number of cells that take up one or more copies of the plasmid per μg of plasmid. If the plasmid contains a reporter gene, transfection efficiency of cells can also be expressed in units of expression of the reporter gene per cell.

The term “replicable vector,” as used herein, is intended to refer to a nucleic acid molecule capable of transporting another nucleic acid to which it has been linked into a cell and providing for amplification of the nucleic acid. One type of vector is a “plasmid”, which refers to a circular double stranded DNA loop into which additional DNA segments may be ligated. Another type of vector is a phage vector. Another type of vector is a viral vector, wherein additional DNA segments may be ligated into the viral genome. Certain vectors are capable of autonomous replication in a host cell into which they are introduced (e.g., bacterial vectors having a bacterial origin of replication and episomal mammalian vectors). Other vectors (e.g., non-episomal mammalian vectors) can be integrated into the genome of a host cell upon introduction into the host cell, and thereby are replicated along with the host genome. In the present specification, “plasmid” and “vector” may be used interchangeably as the plasmid is the most commonly used form of vector. In some embodiments, the vector is a vector that can replicate to high copy number in a cell.

II. Modes for Carrying Out the Invention

Previous attempts to produce infectious clones of parvovirus B19 have been unsuccessful due to deletions in the ITR sequences (Shade et al., 1986, J. Virol., 58:921-936) and the instability of the ITRs in bacterial cells. In addition, parvovirus B19 can be cultured in permissive cells but the amount of virus produced in these cells is very small. There have been no methods or clones of the viral genome that can provide for consistent production of infectious virus. Utilizing the methods of the invention, the genome of parvovirus B19 isolate was cloned and sequenced. A vector was prepared comprising a B19 viral genome and the vector was used to clone the viral genome. The parvovirus B19 clone can be introduced into other cells types (whether permissive or not) to produce infectious virus.

The infectious clone and methods described herein can be utilized in a variety of assays and to develop therapeutic products. The infectious clone is useful for producing infectious virus. An in vitro system for producing infectious virus particles can be used in screening methods to identify agents such as antibodies or antisense molecules that can inhibit viral infectivity or reproduction. The infectious virus and/or infectious virus in a host cell can be utilized to form immunogenic compositions to prepare therapeutic antibodies or vaccine components. Antibodies and primers can be developed to specifically identify different parvovirus B19 isolates. The ability to produce infectious virus in vitro is also useful to develop attenuated strains of the virus that may be utilized in vaccines.

A. Methods of the Invention

One aspect of the invention involves a method of cloning a viral genome that has one or more inverted repeats or secondary structure of nucleic acid that is unstable in cells. A method of the invention comprises introducing the viral genome into a bacterial cell that is deficient in recombinase enzymes such as recA1, end A1, recB, recJ or combinations thereof. The bacterial cells are incubated at a low temperature, for example about 25° C. to 35° C., preferably about 25° C. to 32° C., and more preferably about 28° C. to 31° C., and most preferably about 30° C. The cells are incubated for a time sufficient to allow amplification of the viral genome. Preferably, the incubation time is about 8 to 24 hours, more preferably about 8 to 12 hours. The viral genome is recovered from the bacterial cells.

In some embodiments, the methods of the invention include a method for cloning an infectious parvovirus B19 clone. In an embodiment, the method comprises introducing a replicable vector comprising a parvovirus B19 viral genome or portion thereof into prokaryotic cells that are deficient in major recombination genes, such as for example recA1, endA1, recB and/or recJ or combinations thereof. The cells are incubated at a low temperature for a time sufficient to allow amplification of the vector. The infectious clone is recovered from the prokaryotic cells. Once the infectious clone is prepared it can be introduced into other cell types, whether permissive or not, and provide infectious virus.

Preparing a Clone of the Viral Genome

The infectious clone is comprised of all or a portion of a viral genome of parvovirus B19 and a replicable vector that can provide for amplification of the viral genome in a cell, such as a bacterial cell. In some embodiments, the vector has a bacterial origin of replication. In some embodiments, the vector is a plasmid. In some embodiments, the vector can be selected based on the host cell as well as other characteristics such as compatibility with host cell, copy number, and restriction sites. Vectors that can be used in the invention include, without limitation, pBR322, pProExHTb, pUC19, and pBluescript KS.

The method of cloning a parvovirus genome can be applied to any parvovirus genome. The parvovirus genome includes those obtained from known isolates, those isolated from samples from infected tissues, or parvovirus genomes from any source including those that have been modified. All or a portion of the viral genome can be cloned. In some embodiments, the parvovirus B19 genome is a full-length genome. In other embodiments, a portion of the parvovirus genome comprises or consists of nucleic acid sequence encoding at least one ITR, VP2, NS and the 11 kDa protein in a single replicable vector. The portion of the viral genome is that portion that is sufficient to provide for production of infectious virus. In other embodiments, the parvovirus genome comprises or consists of a nucleic acid encoding an 11R at the 5′ end and an ITR at the 3′ end, VP2, NS and the 11 kDa protein in a single replicable vector. In an embodiment, the B19 genome comprises a polynucleotide encoding an infectious B19 clone having at least 90% nucleic acid sequence identity with SEQ ID NO:5 and/or SEQ ID NO:24. In another embodiment, the B19 genome comprises a nucleic acid sequence of SEQ ID NO:5.

The parvovirus B19 genome preferably comprises one or more ITR sequences. The ITRs include an imperfect palindrome that allows for the formation of a double stranded hairpin with some areas of mismatch that form bubbles. The ITRs serve as a primer for viral replication and contain a recognition site for NS protein that may be required for viral replication and assembling. In some embodiments, the nucleotide sequence that forms the hairpins is retained and conserved. In some embodiments, the location and number of the bubbles or areas of mismatch are conserved as well as the NS binding site. The NS binding site provides for cleavage and replication of the viral genome.

In an embodiment, the parvovirus B19 genome comprises one or more ITR sequences. Preferably, the B19 genome comprises an ITR sequence at the 5′ end and the 3′ end. An ITR may be about 350 nucleotides to about 400 nucleotides in length. An imperfect palindrome may be formed by about 350 to about 370 of the distal nucleotides, more preferably about 360 to about 365 of the distal nucleotides. Preferably the imperfect palindrome forms a double-stranded hairpin. In an embodiment, the ITRs are about 383 nucleotides in length, of which about 365 of the distal nucleotides are imperfect palindromes that form double-stranded hairpins. In another embodiment, the ITRs are about 381 nucleotides in length, of which about 361 of the distal nucleotides are imperfect palindromes that form double-stranded hairpins. In some embodiments, a B19 genome comprises at least 75% of the nucleotide sequence that forms the hairpin in the ITR at the 5′ end and 3′ end of the genome. In other embodiments, the ITRs may have 1 to about 5 nucleotides deleted from each end. In preferred embodiments, the ITR has at least about 94%, more preferably 95%, more preferably 96%, more preferably 97%, more preferably 98%, more preferably about 99%, and more preferably 100% of the sequence of that of viral genome isolated from nature, such as that of SEQ ID NO:5 or SEQ ID:24. In a further embodiment, the ITRs comprise a nucleic acid sequence of SEQ ID NO:1 and/or SEQ ID NO:2. The ITRs may be in the “flip” or “flop” orientation.

The parvovirus genome may have variation due to variation in naturally occurring isolates. For example, isolates of parvovirus B19 from infected tissues can have about 90% sequence identity or greater to that of parvovirus B19 Au (GeneBank accession number M13178; SEQ ID NO:24) In some embodiments, a parvovirus genome has at least 90% sequence identity, more preferably more preferably at least 91%, more preferably at least 92%, more preferably at least 93%, more preferably at least 94%, more preferably at least 95%, more preferably at least 96%, more preferably at least 97%, more preferably at least 98%, more preferably at least 99% or greater sequence identity to that of a parvovirus B19 genome comprising a nucleic acid sequence of parvovirus B19 Au GeneBank accession number M13178; SEQ ID NO:24).

In some cases, alterations or modifications may be made to the nucleic acid sequence of the viral genome of a viral isolate using standard methods to form variant viral genomes. The alterations may be made to add or delete characteristics to the nucleic acid sequence. For example, it may be desirable to add or delete a restriction site or add a sequence that can serve to identify the viral genome. In a specific embodiment, a vector, identified as pB19-M20 comprises a full-length clone of parvovirus B19 having a sequence of SEQ ID NO:5 but with a change at nucleotide 2285 from a cytosine to a thymine, resulting in conversion of BsrI site to a Dde site. In another embodiment, a vector, identified as pB19-4244d comprises a full-length clone of parvovirus B19 having a sequence of SEQ ID NO:5 but with a change to eliminate an XbaI restriction site.

Alternatively it may be desirable to add a nucleic acid sequence that encodes a heterologous polypeptide to the infectious clone. Such a heterologous polypeptide may include tag polypeptides such as poly-histidine (poly-His) or poly-histidine-glycine (poly-His-gly) tags; the flu HA tag polypeptide and its antibody 12CA5 [Field et al., Mol. Cell. Biol., 8:2159-2165 (1988)]; the c-myc tag and the 8F9, 3C7, 6E10, G4, B7 and 9E10 antibodies thereto [Evan et al., Molecular and Cellular Biology, 5:3610-3616 (1985)]; and the Herpes Simplex virus glycoprotein D (gD) tag and its antibody [Paborsky et al., Protein Engineering, 3(6):547-553 (1990)]. Other tag polypeptides include the Flag-peptide [Hopp et al., BioTechnology, 6:1204-1210 (1988)]; the KT3 epitope peptide [Martin et al., Science, 255:192-194 (1992)]; an “-tubulin epitope peptide [Skinner et al., J. Biol. Chem., 266:15163-15166 (1991)]; and the T7 gene 10 protein peptide tag [Lutz-Freyermuth et al., Proc. Natl. Acad. Sci. USA, 87:6393-6397 (1990)] The heterologous polypeptides are combined with viral proteins to form fusion proteins. Epitopes from heterologous proteins may be combined with parvovirus B19 proteins to form fusion proteins useful for immunogenic compositions.

Preferably, the variant viral genome has at least 90% sequence identity, more preferably at least 91%, more preferably at least 92%, more preferably at least 93%, more preferably at least 94%, more preferably at least 95%, more preferably at least 96%, more preferably at least 97%, more preferably at least 98%, more preferably at least 99% or greater sequence identity to that of a parvovirus B19 genome comprising a nucleic acid sequence of SEQ. ID. NO:5. In some embodiments, the parvovirus genome, preferably has 99.2% sequence identity, more preferably 99.3%, more preferably 99.4%, more preferably 99.5%, more preferably 99.6%, more preferably 99.7%, more preferably 99.8%, and more preferably 99.9% or greater sequence identity to that of a parvovirus B19 genome comprising a polynucleotide sequence of SEQ. M. NO:5.

In some embodiments, the B19 genome is cloned by cloning at least two portions of the viral genome into separate vectors and recombining the two portions into a single vector. Preferably, the two portions of the viral genome comprise an ITR at the end of the portion. The portions of the viral genome can be obtained by digesting the genome with a restriction enzyme that cuts the genome at a location between the ITRs. Preferably the restriction enzyme cuts the genome at a location at least about 800 nucleotides from the ITR. The portions may be cut and religated to reduce the vector size and eliminate undesired restriction sites. For example, the B19 genome may be digested with BamHI. The two fragments (right end genome fragment and left end genome fragment) generated by BamHI digestion are ligated into separate BamHI-StuI digested pProEX HTb vectors (Invitrogen-Life Technologies). See, for example, FIG. 3. To reduce the vector size and eliminate undesired restriction sites, clones that contain the right end of the genome (pB19-42d6) may be digested with EcoRV and religated. The full-length genome is generated by digesting the plasmid containing the left end genome fragment (pB19-44) with BamHI and Ecl136II and cloning the fragment containing the left end genome fragment into the BamHI/EheI site of the pB19-42d6 plasmid (FIGS. 3 and 4).

In some embodiments, it may be desirable to achieve a high efficiency of ligation. In that case, it is preferred that at least about 0.25 μg of the viral genome is combined with about 1 μg of the vector, more preferably about 0.25 to about 0.5 μg of viral genome per 1 μg amount of vector. The viral genome can be obtained from serum or infected cells. The isolated virus may be high titer virus and/or concentrated to achieve the amount of viral genome necessary for ligation. In some embodiments, the parvovirus B19 isolated from a sample and used to prepare the clone is present in the sample at about 10⁸ to about 10¹⁴ genome copies/ml of original sample, more preferably about 10⁸ to about 10¹² genome copies/ml of original sample. Virus can be concentrated from serum or infected cells using standard methods known in the art, such as for example, velocity and/or equilibrium density centrifugation using sucrose solutions in low-salt buffer. Preferably, viral genome is concentrated at about 10⁸ to about 10¹⁴ genome copies/100 μl of physiological solution, more preferably about 10⁸ to about 10¹² genome copies/100 μl of physiological solution.

Introducing and Amplifying a Parvovirus B19 Clone in Prokaryotic Cells

According to the method of cloning a viral genome, a vector comprising all or a portion of the viral genome is introduced into a prokaryotic cell. Methods of introducing vectors into cells are known to those of skill in the art and include transformation methods such as calcium salt precipitation, liposomes, polyethylene glycol/DMSO, electroporation and electro nuclear transport. In some embodiments, the vector is introduced into bacterial cells by electroporation.

The bacterial cells are preferably deficient in recombinase enzymes such as recA1, endA1, recB, recJ, or combinations thereof. In some embodiments, the transformed bacterial cells are preferably E. coli cells. In an embodiment, the E. coli cells are McrA-, McrCB-, McrF-, Mrr-, HsdR-, and endA deficient. In another embodiment, the E. coli cells comprise a genotype of e14-(McrA-)Δ(mcrCB-hsdSMR-mrr)171 endA1 supE44 thi-1 gyrA96 relA1 lac recB recJ sbcC umuC::Tn5 (Kanr) uvrC [F′ proAB lacIqZ.M15 Tn10 (Tetr)]; F⁻ mcrA Δ(mrr-hsdRMS-mcrBC) recA1 endA1 lon gyrA96 thi-1 supE44 relA1λ⁻ Δ(lac-proAB); or F⁻ mcrA Δ(mcrBC-hsdRMS-mrr) recA1 endA1 lon gyrA96 thi supE44 relA1λ⁻ Δ(lac-proAB). In another embodiment, the E. coli cells are cells, such as for example SURE2® cells (Stratagene, La Jolla, Calif.); Stbl2 cells or Stble 4 cells (Invitrogen, Carlsbad, Calif.).

In an embodiment, the incubation temperature for the transformed prokaryotic cells is about 25° C. to about 35° C., preferably, about 25° C. to about 32° C., more preferably about 30° C. to about 32° C., more preferably about 30° C. The prokaryotic cells preferably are plated immediately following introduction of the viral genome and incubated for a sufficient time to allow for amplification of the viral genome, preferably, from about 12 to about 24 hours, more preferably from about 16 to about 18 hours. The clone can be recovered from the cells using standard methods. Infectious clones are those that can produce viral DNA, proteins or particles when introduced into other cell types.

Introducing the Infectious Clone into Other Cell Types

Another aspect of the invention, provides a method of producing an infectious clone or infectious viral particles of parvovirus B19 in a eukaryotic cell. This method can also be utilized to identify and/or confirm that the parvovirus B19 clone produced is infectious. After the clone is amplified in a bacterial cell and recovered, the infectious clone may be introduced into other cell types (whether permissive or not) to identify whether the clone can produce infectious virus or for the production of infectious virus. The method provides for production of infectious virus in vitro. Utilizing an infectious clone allows introduction of the viral genome into a cell without the need for entry mediated by viral proteins such as the capsid protein. The method comprises introducing a vector comprising an infectious clone of parvovirus B19 or all or a portion of a viral genome into a eukaryotic cell and incubating the cell for a sufficient time to produce infectious virus and optionally, detecting production of infectious virus. The method of identifying an infectious clone comprises introducing a vector comprising all or a portion of a viral genome into a eukaryotic cell; incubating the cell for a sufficient time to produce infectious virus; and detecting production of infectious virus.

In some embodiments, a high efficiency of introduction of the vector into eukaryotic cells is desired. Preferably, the method of introduction employed achieves a transfection efficiency of at least about 15% to 100% efficiency, more preferably about 30 to 50% efficiency. The method is also selected to minimize cytotoxicity to the cells. Preferably, about 20% or greater of the cells are viable and more preferably about 50% of the cells or greater. In some embodiments, the vector may be cut with one or more restriction enzymes to enhance viral replication.

In an embodiment, eukaryotic cells are transfected with an electric current. Methods of transfecting eukaryotic cells utilizing an electric current are known in the art, such as for example, electroporation (Sambrook, et al., 1989, Molecular Cloning, A Laboratory Manual, Vols. 1-3, Cold Spring Harbor Press, Cold Spring Harbor, N.Y. or Davis et al., 1986, Basic Methods in Molecular Biology) and electrical nuclear transport (U.S. 20040014220).

In an embodiment, the eukaryotic cells are transfected by electrical nuclear transport. The cells are exposed to an electrical pulse comprising a field strength of about 2 kV/cm to about 10 kV/cm, a duration of about 10 μsec to about 200 μsec, and a current of at about 1 A to about 2.5 A followed by a current flow of about 1 A to about 2.5 A for about 1 msec to about 50 msec. A buffer suitable for use in electrical nuclear transport comprises 0.42 mM Ca(NO₃)₂, 5.36 mM KCl, 0.41 mM MgSO₄, 103 mM NaCl, 23.8 mM NaHCO₃, 5.64 mM Na₂HPO₄, 11.1 mM d(+) glucose, 3.25 μM glutathione, 20 mM Hepes, and pH 7.3. Following transformation, the permissive cells may be incubated for about 10 min at 37° C. before being plated in prewarmed (37° C.) culture medium with serum and incubated at 37° C.

Commercially available devices and buffer systems for electrical nuclear transport, such as for example the AMAXA CELL LINE NUCLEOFECTOR™ system (Amaxa Biosystems Inc., Nattermannallee, Germany; www-amaxa-com), have been customized to transduce specific types of eukaryotic cells such as, for example, UT7/Epo cells UT7/Epo-S1 cells. In an embodiment, UT7/Epo cells or UT7/Epo-S1 cells are transfected using NUCLEOFECTOR™ reagent R and program T-20 on the NUCLEOFECTOR™ device according to the manufacturer's instructions (Amaxa Biosystems Inc., Nattermannallee, Germany).

The eukaryotic cells include, but are not limited to, erythroid progenitor cells, fetal liver cells, UT7/EPO cells, UT7/EPO-S1 cells, or KU812Ep6 cells. In an embodiment, permissive cells include, but are not limited to, primary erythroid progenitor cells from bone marrow and blood; megakaryoblast cells, fetal liver cells; UT7/Epo cells, UT7/Epo-S1 cells, KU812Ep6 cells, JK-1 and MB-O2. Other eukaryotic cell types may also be utilized including 293 cells, CHO cells, Cos cells, Hela cells, BHK cells and SF9 cells.

The cells may be incubated in culture medium following introduction of the vector comprising a parvovirus B19 viral genome or plated in culture medium immediately following transfection. The cells may be incubated for about 10 min to about 30 min at about 25° C. to about 37° C., more preferably about 30° C. to about 37° C., more preferably 37° C. before plating the cells. Once plated, the cells are incubated under conditions sufficient to provide for production of infectious virus. In some embodiments, the cells are incubated at 37° C. for about 2 to about 4 hours, more preferably at least about 6 hours, more preferably at least about 12 hours, more preferably at least about 18 hours, more preferably at least about 24 hours. In an embodiment, the cells are incubated for about 72 hours post-transfection. Infectious virus particles can be isolated or recovered from cell lysates.

To determine if B19 virus produced by the methods of the invention is infectious, supernatants prepared from cell lysates of the cells can be used to infect non-transfected cells. In an embodiment, the non-transfected cells are UT7/Epo-S1 cells. Production of infectious B19 virus by the methods of the invention may be detected by analyzing the infected cells for spliced transcripts of B19 genes. Preferably the spliced transcripts are spliced capsid transcripts encoding, for example, VP1 or VP2. In an embodiment, infectious B19 is identified by contacting cells with supernatant from the transformed cells and analyzing the contacted cells for B19 spliced transcripts. Detection of spliced capsid transcripts indicates the parvovirus B19 is infectious. Production of infectious B19 virus may be detected by analyzing the infected cells for B19 viral proteins. Preferably the B19 viral proteins are capsid proteins, such as for example VP1 and VP2. In an embodiment, infectious parvovirus B19 virus is identified by contacting cells with supernatant from the transfected cells and analyzing the contacted cells for B19 viral proteins. Detection of B19 capsid proteins indicates the parvovirus B19 is infectious. In another embodiment, in vitro neutralization assays can be performed to test whether neutralizing monoclonal antibodies against parvovirus B19 capsids are able to block the infection caused by the cell lysates of transfected cells. Blocking of infectivity by neutralizing antibodies indicates the virus is infectious.

B. Infectious Parvovirus B19 Clones

The invention also provides infectious B19 clones and polynucleotides encoding the infectious clones. The infectious clones may be produced by the methods of the invention. The infectious clone is comprised of all or a portion of a viral genome of parvovirus B19 and a replicable vector that can provide for amplification of the viral genome in a bacterial cell, in some embodiments, the vector has a bacterial origin of replication. In some embodiments, the vector is a plasmid. In some embodiments, the vector can be selected based on the host cell as well as other characteristics such as compatibility with host cell, copy number, and restriction sites. Vectors that can be used in the invention include, without limitation, pBR322, pProExHTb, pUC19, and pBluescript KS. Preferably, the vector provides for high copy number of the infectious clone in bacterial cells, eg about 50-100 copies per cell. Several embodiments of the infectious clones are described in the Examples and the Figures.

The method of cloning a parvovirus genome can be applied to any parvovirus genome. Thus, an infectious clone can comprise a parvovirus genome obtained from known isolates, those isolated from samples from infected tissues, or parvovirus genomes from any source including those genomes that have been modified. All or a portion of the viral genome can be cloned. In some embodiments, the parvovirus B19 genome is a full length genome. In other embodiments, a portion of the parvovirus genome comprises or consists of nucleic acid sequence encoding at least one ITR, VP2, NS and the 11 kDa protein in a single replicable vector. The portion of the viral genome is that portion that is sufficient to provide for production of infectious virus. In other embodiments, the parvovirus genome comprises or consists of a nucleic acid encoding an ITR at the 5′ end and an ITR at the 3′ end, VP2, NS and the 11 kDa protein in a single replicable vector. In an embodiment, the B19 genome comprises a polynucleotide encoding an infectious B19 clone having at least 90% nucleic acid sequence identity with SEQ ID NO:5. In another embodiment, the B19 genome comprises a nucleic acid sequence of SEQ ID NO:5.

The parvovirus B19 genome preferably comprises one or more ITR sequences. “ITR” or “ITR sequence” refers to an inverted terminal repeat of nucleotides in a nucleic acid such as a viral genome. The ITRs include an imperfect palindrome that allows for the formation of a double stranded hairpin with some areas of mismatch that form bubbles. The ITRs serve as a primer for viral replication and contain a recognition site for NS protein that may be required for viral replication and assembling. In some embodiments, the location and number of the bubbles or areas of mismatch are conserved as well as the NS binding site. The NS binding site provides for cleavage and replication of the viral genome. In an embodiment, the parvovirus B19 genome comprises one or more ITR sequences. Preferably, the B19 genome comprises an ITR sequence at the 5′ end and the 3′ end. An ITR may be about 350 nucleotides to about 400 nucleotides in length. An imperfect palindrome may be formed by about 350 to about 370 of the distal nucleotides, more preferably about 360 to about 365 of the distal nucleotides. Preferably the imperfect palindrome forms a double-stranded hairpin. In an embodiment, the ITRs are about 383 nucleotides in length, of which about 365 of the distal nucleotides are imperfect palindromes that form double-stranded hairpins. In another embodiment, the ITRs are about 381 nucleotides in length, of which about 361 of the distal nucleotides are imperfect palindromes that form double-stranded hairpins. In some embodiments, a B19 genome comprises at least 75% of the nucleotide sequence that forms the hairpin in the ITR at the 5′ end and 3′ end of the genome. In other embodiments, the ITRs may have 1 to about 5 nucleotides deleted from each end. In preferred embodiments, the ITR has at least about 94%, more preferably 95%, more preferable 96%, more preferably 97%, more preferably 98%, more preferably about 99%, and more preferably 100% of the sequence of that of viral genome isolated from nature, such as that of SEQ ID NO:5 or SEQ ID:24. In a further embodiment, the ITRs comprise a nucleic acid sequence of SEQ ID NO:1 and/or SEQ ID NO:2. The ITRs may be in the “flip” or “flop” orientation.

The parvovirus genome may have variation due to variation in naturally occurring isolates. For example, isolates of parvovirus B19 from infected tissues can have about 90% sequence identity or greater to that of parvovirus B19-Au (GeneBank Accession No. M13178; SEQ ID NO:24). In some embodiments, a parvovirus genome has at least 90% sequence identity, more preferably more preferably at least 91%, more preferably at least 92%, more preferably at least 93%, more preferably at least 94%, more preferably at least 95%, more preferably at least 96%, more preferably at least 97%, more preferably at least 98%, more preferably at least 99% or greater to that of a parvovirus B19 genome comprising a nucleic acid sequence of parvovirus B19 Au (GeneBank Accession No. M13178; SEQ ID NO:24).

In some cases, alterations or modifications may be made to the nucleic acid sequence of the viral genome using standard methods. The alterations may be made to add or delete characteristics to the nucleic acid sequence. For example, it may be desirable to add or delete a restriction site or add a sequence that can serve to identify the viral genome. In a specific embodiment, a vector, identified as pB19-M20 comprises a full length clone of parvovirus B19 having a sequence of SEQ ID NO:5 but with a change at nucleotide 2285 from a cytosine to a thymine, resulting in conversion of BsrI site to a Dde site. In another embodiment, a vector, identified as pB19-4244d comprises a full length clone of parvovirus B19 having a sequence of SEQ ID NO:5 but with a change to eliminate an XbaI restriction site.

Alternatively it may be desirable to add a nucleic acid sequence that encodes a heterologous polypeptide to the infectious clone. Such a heterologous polypeptide may include tag polypeptides such as poly-histidine (poly-His) or poly-histidine-glycine (poly-His-gly) tags; the flu HA tag polypeptide and its antibody 12CA5 [Field et al., Mol. Cell. Biol., 8:2159-2165 (1988)]; the c-myc tag and the 8F9, 3C7, 6E10, G4, B7 and 9E10 antibodies thereto [Evan et al., Molecular and Cellular Biology, 5:3610-3616 (1985)]; and the Herpes Simplex virus glycoprotein D (gD) tag and its antibody [Paborsky et al., Protein Engineering, 3(6):547-553 (1990)]. Other tag polypeptides include the Flag-peptide [Hopp et al., BioTechnology, 6:1204-1210 (1988)]; the KT3 epitope peptide [Martin et al., Science, 255:192-194 (1992)]; an “-tubulin epitope peptide [Skinner et al., J. Biol. Chem., 266:15163-15166 (1991)]; and the T7 gene 10 protein peptide tag [Lutz-Freyermuth et al., Proc. Natl. Acad. Sci. USA, 87:6393-6397 (1990)]. Heterologous polypeptides are combined with viral proteins to form fusion proteins. Epitopes from other proteins may be combined with parvovirus B19 proteins to form fusion proteins useful as immunogenic compositions.

Preferably, the viral genome has at least 90% sequence identity, more preferably at least 91%, more preferably at least 92%, more preferably at least 93%, more preferably at least 94%, more preferably at least 95%, more preferably at least 96%, more preferably at least 97%, more preferably at least 98%, more preferably at least 99% or greater to that of a parvovirus B19 genome comprising a nucleic acid sequence of SEQ. ID. NO:5. In some embodiments, the parvovirus genome, preferably has 99.2% sequence identity, more preferably 99.3%, more preferably 99.4%, more preferably 99.5%, more preferably 99.6%, more preferably 99.7%, more preferably 99.8%, and more preferably 99.9% or greater sequence identity to that of a parvovirus B19 genome comprising a nucleic acid sequence of SEQ. ID. NO:5.

In some embodiments, the B19 genome is cloned by, cloning at least two portions of the viral genome into separate vectors and recombining the two portions into a single vector. Preferably, two portions of the viral genome comprise an ITR at the end of the portion. The portions of the viral genome can be obtained by digesting the genome with a restriction enzyme that cuts the genome at a location between the ITRs. Preferably the restriction enzyme cuts the genome at a location at least about 800 nucleotides from the ITR. The portions may be cut and religated to reduce the vector size and eliminate undesired restriction sites. For example, the B19 genome may be digested with BamHI. The two fragments (right end genome fragment and left end genome fragment) generated by BamHI digestion are ligated into separate BamHI-StuI digested pProEX HTb vectors (Invitrogen-Life Technologies). See, for example, FIG. 3. To reduce the vector size and eliminate undesired restriction sites, clones that contain the right end of the genome (pB19-42d6) may be digested with EcoRV and religated. The full-length genome is generated by digesting the plasmid containing the left end genome fragment (pB19-44) with BamHI and Ecl136II and cloning the fragment containing the left end genome fragment into the BamHI/EheI site of the pB19-42d6 plasmid (FIGS. 3 and 4).

In some embodiments, it may be desirable to achieve a high efficiency of ligation. In that case, it is preferred that at least about 0.25 μg of the viral genome is combined with about 1 μg of the vector, more preferably about 0.25 to about 0.5 μg or greater of viral genome per 1 μg amount of vector. The viral genome can be obtained from serum or infected cells. The isolated virus may be high titer virus and/or concentrated to achieve the amount of viral genome necessary for ligation. In some embodiments, the parvovirus B19 isolated from a sample and used to prepare the clone is present in the sample at about 10⁸ to about 10¹⁴ genome copies/ml of original sample, more preferably about 10⁸ to about 10¹² genome copies/ml of original sample. Virus can be concentrated from serum or infected cells using standard methods known in the art, such as for example, velocity and/or equilibrium density centrifugation using sucrose solutions in low-salt buffer. Preferably, viral genome is concentrated at about 10⁸ to about 10¹⁴ genome copies/100 μl of physiological solution, more preferably about 10⁸ to about 10¹² genome copies/100 μl of physiological solution.

The infectious clone is preferably stable and can be passaged through bacterial cell culture without loss of functional ITRs. The stability can be determined by introducing the infectious clone into bacterial cells and subcloning and religating several times. In preferred embodiments, the clone can be passaged in bacterial cells at temperatures ranging from about 30° C. to about 37° C. at least about 10 times without substantial loss of ITR nucleic acid sequence.

C. Recombinant Methods, Vectors, and Host Cells

The infectious B19 clones of the invention are produced by synthetic and recombinant methods. Accordingly, the invention relates to polynucleotides encoding the infectious B19 clones of the invention (such as for example a B19 genome) and host cells containing the infectious clone, as well as methods of making such vectors and host cells by recombinant methods.

The B19 clones of the invention may be synthesized or prepared by techniques well known in the art. Some nucleotide sequences for parvovirus B19 genomes are known and readily available, for example, on the Internet at GenBank (accessible at www-ncbi-nlm-nihgov/entrez). The nucleotide sequences encoding the B19 clones of the invention may be synthesized or amplified using methods known to those of ordinary skill in the art including utilizing DNA polymerases in a cell free environment.

The B19 clones of the invention can be produced from viral isolated obtained from biological samples. The polynucleotides may be produced by standard recombinant methods known in the art, such as polymerase chain reaction (Sambrook, et al., 1989, Molecular Cloning, A Laboratory Manual, Vols. 1-3, Cold Spring Harbor Press, Cold Spring Harbor, N.Y. Methods of altering or modifying nucleic acid sequences are also known to those of skill in the art.

As described herein in the methods of the invention, the B19 genome may be assembled from polymerase chain reaction cassettes sequentially cloned into a vector containing a selectable marker for propagation in a host. Such markers include dihydrofolate reductase or neomycin resistance for eukaryotic cell culture and tetracycline or ampicillin resistance genes for culturing in E. coli and other bacteria.

The polynucleotide may be inserted into a replicable vector for cloning (amplification of the DNA) as described in the methods herein. Various vectors are publicly available. The vector may, for example, be in the form of a plasmid, cosmid, viral particle, or phage. The appropriate nucleic acid sequence may be inserted into the vector by a variety of procedures. In general, DNA is inserted into an appropriate restriction endonuclease site(s) using techniques known in the art. Vector components generally include, but are not limited to, one or more of a signal sequence, an origin of replication, one or more marker genes, an enhancer element, a promoter, and a transcription termination sequence. Construction of suitable vectors containing one or more of these components employs standard ligation techniques that are known to the skilled artisan.

Examples of suitable replicable vectors include, without limitation, pCR-Blunt II TOPO vector (Invitrogen, San Diego, Calif.), pProEX Htb vector (Invitrogen, San Diego, Calif.), and pBR332 (Deiss et al., 1990, Virology, 175:247-254), and pBluescipt SK. The polynucleotide can be operably linked to an appropriate promoter such as, for example, the parvovirus B19 p6 promoter. Additional suitable promoters are known in the art such as SV40 or CMV. The replicable vectors may further contain sites for transcription initiation, transcription termination, and a ribosome binding site for translation.

In an embodiment, the full length B19 genome is cloned by digesting the genome with a restriction enzyme that cuts the genome into two fragments, cloning the two fragments, and religating the two fragments to form the full-length genome. The B19 genome may be digested, for example, with BamHI. The two fragments (right end genome fragment and left end genome fragment) generated by BamHI digestion are ligated into separate BamHI-StuI digested pProEX HTb vectors (Invitrogen-Life Technologies). See, for example, FIG. 3. To reduce the vector size and eliminate undesired restriction sites, clones that contain the right end of the genome (pB19-42d6) may be digested with EcoRV and religated. The full-length genome is generated by digesting the plasmid containing the left end genome fragment (pB19-44) with BamHI and Ecl136II and cloning the fragment containing the left end genome fragment into the BamHI/EheI site of the pB19-42d6 plasmid (FIGS. 3 and 4).

Introduction of a recombinant vector comprising a B19 genome into a host cell, such as for example a bacterial cell or eukaryotic cell, can be affected by calcium phosphate transfection, DEAE-dextran mediated transfection, cationic lipid-mediated transfection, electroporation, electrical nuclear transport, chemical transduction, electrotransduction, infection, or other methods. Such methods are described in standard laboratory manuals such as Sambrook, et al., 1989, Molecular Cloning, A Laboratory Manual, Vols. 1-3, Cold Spring Harbor Press, Cold Spring Harbor, N.Y. or Davis et al., 1986, Basic Methods in Molecular Biology. Commercial transfection reagents, such as Lipofectamine (Invitrogen, Carlsbad, Calif.) and FuGENE 6™ (Roche Diagnostics, Indianapolis, Ind.), are also available. Preferably transfection efficiency of the host cells is about 15% or greater, more preferably about 20% or greater, more preferably about 30% or greater, more preferably about 40% or greater, more preferably about 50% or greater, more preferably about 70% or greater

In an embodiment, eukaryotic cells are transfected with an electric current. Methods of transfecting eukaryotic cells utilizing an electric current are known in the art, such as for example, electroporation (Sambrook, et al., 1989, Molecular Cloning, A Laboratory Manual, Vols. 1-3, Cold Spring Harbor Press, Cold Spring Harbor, N.Y. or Davis et al., 1986, Basic Methods in Molecular Biology) and electrical nuclear transport (U.S. 20040014220).

In an embodiment, a eukaryotic cell is transfected by electrical nuclear transport. The permissive cells are exposed to an electrical pulse comprising a field strength of about 2 kV/cm to about 10 kV/cm, a duration of about 10 μsec to about 200 μsec, and a current of at about 1 A to about 2.5 A followed by a current flow of about 1 A to about 2.5 A for about 1 msec to about 50 msec. A buffer suitable for use in electrical nuclear transport comprises 0.42 mM Ca(NO₃)₂, 5.36 mM KCl, 0.41 mM MgSO₄, 103 mM NaCl, 23.8 mM NaHCO₃, 5.64 mM Na₂HPO₄, 11.1 mM d(+) glucose, 3.25 μM glutathione, 20 mM Hepes, and pH 7.3. Following transformation, the permissive cells may be incubated for about 10 min at 37° C. before being plated in prewarmed (37° C.) culture medium with serum and incubated at 37° C.

Commercially available devices and buffer systems for electrical nuclear transport, such as for example the AMAXA CELL LINE NUCLEOFECTOR™ system (Amaxa Biosystems Inc., Nattermannallee, Germany; www-amaxa-com), have been customized to transduce specific types of eukaryotic cells such as, for example, UT7/Epo cells UT7/Epo-S1 cells. In an embodiment, UT7/Epo cells or UT7/Epo-S1 cells are transfected using NUCLEOFECTOR™ reagent R and program T-20 on the NUCLEOFECTOR™ device according to the manufacturer's instructions (Amaxa Biosystems Inc., Nattermannallee, Germany).

D. Uses

The infectious clone and methods described herein can be utilized in a variety of assays and to develop therapeutic products. As discussed previously, methods for consistently obtaining infectious virus in cell culture were not previously known. An in vitro system for producing infectious virus particles can be used in screening methods to identify agents such as antibodies or antisense molecules that can inhibit viral infectivity or reproduction. The infectious virus and/or infectious virus in a host cell can be utilized to form immunogenic compositions to prepare therapeutic antibodies or vaccine components. Antibodies and primers can be developed to specifically identify different parvovirus B19 isolates. The ability to produce infectious virus consistently in vitro is also useful to produce attenuated virus that may be used in a vaccine.

The infectious B19 clones of the invention are useful in diagnostic assays. The presence or absence of an antibody in a biological sample that binds to a B19 clone of the invention can be determined using standard methods. Alternatively, the presence or absence of B19 parvovirus in a biological sample can be determined can be determined using PCR primers specific for nucleic acids encoding an infectious clone of the invention to amplify any parvovirus B19 DNA that may be present in the sample. Several primers have been described in the Examples. The primers and antibodies can be developed to specifically identify different viral isolates based on difference in nucleic acid or protein sequences.

The infectious B19 clones of the invention are also useful to produce antibodies to parvovirus B19. The antibodies are useful in diagnostic assays for detecting the presence of parvovirus B19 in a biological sample. Methods for developing antibodies are described below. One aspect of the invention provides a method for screening for parvovirus B19 infection, comprising contacting a biological sample with an anti-parvovirus B19 antibody and assaying the biological sample for anti-parvovirus B19 antibody binding. The antibodies, preferably recognize a particular isolate.

The invention also provides methods for screening for antibodies that may inhibit or antagonize B19 infection of permissive cells. The antagonist effect of anti-parvovirus B19 antibodies may determined by analyzing cells for B19 capsid proteins or B19 spliced capsid transcripts as described above. Antagonist antibodies can be prepared and screened as described below.

The infectious parvovirus B19 clone and/or host cells comprising the clone can be used as immunogenic compositions to prepare vaccine components and/or to develop antibodies that can be used in diagnostic or other assays. For example, host cell cultures comprising the parvovirus B19 clone can be heat inactivated and used as an immunogen. Passaging of an infectious clone in vitro can provide an attenuated strain of parvovirus B19 useful in vaccine compositions.

E. Production of Antibodies

1. Polyclonal Antibodies

Polyclonal antibodies to infectious B19 clones of the invention of the invention are preferably raised in animals by multiple subcutaneous (sc) or intraperitoneal (ip) injections of the relevant antigen and an adjuvant. The relevant antigen may be, for example, one or more B19 clones of the invention or one or more B19 proteins, such as NS, VP1, VP2, 11-kDa protein, 7.5-kDa protein, and/or protein X, derived from an infectious clone of the infection. It may be useful to conjugate the relevant antigen to a protein that is immunogenic in the species to be immunized, e.g., keyhole limpet hemocyanin, serum albumin, bovine thyroglobulin, or soybean trypsin inhibitor using a bifunctional or derivatizing agent, for example, maleimidobenzoyl sulfosuccinimide ester (conjugation through cysteine residues), N-hydroxysuccinimide (through lysine residues), glutaraldehyde, succinic anhydride, SOCl₂, or R¹N═NR, where R and R¹ are different alkyl groups.

Animals are immunized against the antigen, immunogenic conjugates, or derivatives by combining, e.g., 100 μg or 5 μg of the protein or conjugate (for rabbits or mice, respectively) with 3 volumes of Freund's complete adjuvant and injecting the solution intradermally at multiple sites. One month later the animals are boosted with ½ to 1/10 the original amount of peptide or conjugate in Freund's complete adjuvant by subcutaneous injection at multiple sites. Seven to 14 days later the animals are bled and the serum is assayed for antibody titer. Animals are boosted until the titer plateaus. Preferably, the animal is boosted with the conjugate of the same antigen, but conjugated to a different protein and/or through a different cross-linking reagent. Conjugates also can be made in recombinant cell culture as protein fusions. Also, aggregating agents such as alum are suitably used to enhance the immune response.

In an alternative embodiment, the animals are immunized with a recombinant adenovirus vector expressing one or more viral proteins derived from an infectious clone of the invention, such as for example VP1 and/or VP2, followed by booster immunizations with the viral proteins. The polyclonal antibodies generated by the immunizations may undergo a screen for B19 antagonist activity. Preferably, antibodies to an infectious B19 clone of the invention inhibit the negative effect of B19 on erythocyte production. In an embodiment, antibodies that specifically bind a B19 clone encoded by a polynucleotide comprising a nucleic acid sequence of SEQ ID NO:5 inhibits infection of permissive cells.

The polyclonal antibodies are also screened by enzyme-linked immunoabsorbent assay (ELISA) to characterize binding. The antigen panel includes NS, VP1, VP2, 11-kDa protein, 7.5-kDa protein, and protein X. Animals with sera samples that test positive for binding to one or more experimental antigens in the panel are candidates for use in monoclonal antibody production. The criteria for selection for monoclonal antibody production is based on a number of factors including, but not limited to, binding patterns against a panel of B19 viral proteins.

2. Monoclonal Antibodies

Monoclonal antibodies to an infectious B19 clone of the invention may be made using the hybridoma method first described by Kohler et al., Nature, 256:495 (1975), or may be made by recombinant DNA methods (U.S. Pat. No. 4,816,567).

In the hybridoma method, a mouse or other appropriate host animal, such as a hamster or macaque monkey, is immunized as described above to elicit lymphocytes that produce or are capable of producing antibodies that will specifically bind to a B19 clone of the invention or viral proteins derived from a B19 clone of the invention used for immunization. Alternatively, lymphocytes may be immunized in vitro. Lymphocytes then are fused with myeloma cells using a suitable fusing agent, such as polyethylene glycol, to form a hybridoma cell (Goding, Monoclonal Antibodies: Principles and Practice, pp. 59-103 (Academic Press, 1986)).

The hybridoma cells are than seeded and grown in a suitable culture medium that preferably contains one or more substances that inhibit the growth or survival of the unfused, parental myeloma cells. For example, if the parental myeloma cells lack the enzyme hypoxanthine guanine phosphoribosyl transferase (HGPRT or HPRT), the culture medium for the hybridomas typically will include hypoxanthine, aminopterin, and thymidine (HAT medium), which substances prevent the growth of HGPRT-deficient cells.

Preferred myeloma cells are those that fuse efficiently, support stable high-level production of antibody by the selected antibody-producing cells, and are sensitive to a medium such as HAT medium. Among these, preferred myeloma cell lines are murine myeloma lines, such as those derived from MOPC-21 and MPC-11 mouse tumors available from the Salk Institute Cell Distribution Center, San Diego, Calif. USA, and SP-2 or X63-Ag8-653 cells available from the American Type Culture Collection, Rockville, Md. USA. Human myeloma and mouse-human heteromyeloma cell lines also have been described for the production of human monoclonal antibodies (Kozbor, J. Immunol., 133:3001 (1984); Brodeur et al., Monoclonal Antibody Production Techniques and Applications, pp. 51-63 (Marcel Dekker, Inc., New York, 1987)).

Culture medium in which hybridoma cells are growing is assayed for production of monoclonal antibodies directed against the antigen and HIV Env. Preferably, the binding specificity of monoclonal antibodies produced by hybridoma cells is determined by immunoprecipitation or enzyme-linked immunoabsorbent assay (ELISA).

After hybridoma cells are identified that produce antibodies of the desired specificity, affinity, and/or activity, the clones may be subcloned by limiting dilution procedures and grown by standard methods (Goding, Monoclonal Antibodies: Principles and Practice, pp. 59-103 (Academic Press, 1986)). Suitable culture media for this purpose include, for example, D-MEM or RPMI-1640 medium. In addition, the hybridoma cells may be grown in vivo as ascites tumors in an animal.

The monoclonal antibodies secreted by the subclones are suitably separated from the culture medium, ascites fluid, or serum by conventional immunoglobulin purification procedures such as, for example, protein A-Sepharose, hydroxylapatite chromatography, gel electrophoresis, dialysis, or affinity chromatography.

The monoclonal antibodies are characterized for specificity of binding using assays as described previously. Antibodies can also be screened for antagonist activity as described previously.

3. Human or Humanized Antibodies

Humanized forms of non-human (e.g., murine) antibodies are chimeric antibodies that contain minimal sequence derived from non-human immunoglobulin. For the most part, humanized antibodies are human immunoglobulins (recipient antibody) in which residues from a CDR of the recipient are replaced by residues from a CDR of a non-human species (donor antibody) such as mouse, rat, rabbit or nonhuman primate having the desired specificity, affinity, and capacity. Useful non-human antibodies are monoclonal antibodies that bind specifically to parvovirus B19. Useful non-human antibodies also include antibodies that inhibit B19 infection of permissive cells. In some instances, framework region (FR) residues of the human immunoglobulin are replaced by corresponding non-human residues. Furthermore, humanized antibodies may comprise residues that are not found in the recipient antibody or the donor antibody. These modifications may be made to improve antibody affinity or functional activity. In general, the humanized antibody will comprise substantially all of at least one, and typically two, variable domains, in which all or substantially all of the hypervariable regions correspond to those of a non-human immunoglobulin and all or substantially all of the FRs are those of a human immunoglobulin sequence. The humanized antibody optionally will also comprise at least a portion of an immunoglobulin constant region (Fc), typically that of a human immunoglobulin. For further details, see Jones et al., Nature 321:522-525 (1986); Riechmann et al., Nature 332:323-329 (1988); and Presta, Curr. Op. Struct. Biol. 2:593-596 (1992). See also the following review articles and references cited therein: Vaswani and Hamilton, Ann. Allergy, Asthma & Immunol. 1:105-115 (1998); Harris, Biochem. Soc. Transactions 23:1035-1038 (1995); Hurle and Gross, Curr. Op. Biotech 5:428-433 (1994).

Human antibodies that specifically bind and/or antagonize parvovirus B19 can also be made using the transgenic mice available for this purpose or through use of phage display techniques.

An in vitro system for producing infectious virus particles can be used in screening methods to identify agents such as antibodies or antisense molecules that can inhibit viral infectivity or reproduction. A screening method comprises introducing the viral genome of an infectious clone of parvovirus B19 into a cell and contacting the cells with a potential inhibitory agent, and determining whether the inhibitory agent inhibits infectivity or replication of the viral genome in the cells. Methods for detecting infectivity and replication of the viral genome have been described herein. Potential inhibitory agents include antibodies and anti sense molecules.

The ability to produce infectious parvovirus in vitro may allow for the development of a vaccine or vaccine components. A vaccine can be comprised of heat inactivated virus or attenuated virus. Inactivated virus can be prepared from production of infectious clones using methods known to those of skill in the art. Attenuated virus can be obtained by serially passaging the virus under conditions that make the virus non pathological to humans. The attenuated virus is preferably passaged through a cell and under certain conditions that provide for an altered virus that is less pathological to humans. Vaccine components can also include one or more of the parvovirus proteins or parvovirus proteins combined with epitopes from other infectious agents.

All publications, patents, and patent applications cited herein are hereby incorporated in their entirety by reference. The following examples are provided for illustrative purposes only, and are in no way intended to limit the scope of the present invention.

Example 1 Cloning and Sequencing of Parvovirus B19 Isolate J35

Introduction

The nucleotide sequence of B19 was originally established by sequencing a viral isolate designated pvbaua obtained from the serum of a child with homozygous sickle cell disease (Shade et al. 1986, J. Virol., 58: 921-936). Subsequently, many B19 isolates have been sequenced by multiple methods (Erdman et al. 1996, J. Gen. Virol., 77: 2767-2774). Following alignment of the sequences, there is a 6% divergence amongst the various isolates (Heegaard & Brown, 2002, Clin. Microbiol. Rev., 15: 485-505). The single nonstructural protein (NS1) gene is highly conserved, and the two capsid proteins, VP1 and VP2, occasionally have a greater variability of 2-3% (Hemauer et al. 1996, J. Gen. Virol., 77: 1781-1785; Mori et al. 1987, J. Gen. Virol, 68: 2797-2806).

There is no animal model for B19, and virus can only be grown in culture with difficulty (Heegaard & Brown, 2002). Parvovirus B19 exhibits a selective tropism for erythroid progenitor cells, and can only be cultured in primary erythroid progenitor cells from bone marrow, blood, or fetal liver cells, megakaryoblast cells, UT7/Epo cells, UT7/Epo-S1 cells, KU812Ep6 cells, JK-1 cells, and MB-O2 cells. (Ozawa et al., 1986; Brown et al., 1991; Yaegashi et al., 1989; Komatsu et al., 1993; Shimomura et al.; 1992 Miyagawa et al., 1999). These series of examples establish a method of producing an infectious clone for parvovirus B19.

Methods

Parvovirus B19 (J35) was obtained from the serum of a child with sickle cell anemia undergoing aplastic crisis and sent to NIH for diagnostic purposes. The serum was found by dot blot assay (Nguyen et al., 2002) to contain approximately 10¹² genome copies of B19/mL. UT7/Epo-S1 cells (Shimomura et al., 1992) (maintained in Iscove's modified Dulbecco's medium (IMDM) containing 10% fetal calf serum, 2 U/ml recombinant human erythropoietin (Amgen, Thousand Oaks, Calif.), and antibiotics at 37° C. in 5% CO₂) were infected with the J35 serum containing high titer B19 virus (Nguyen et al., 2002). DNA was extracted by the DNeasy® method (Qiagen Inc, Valencia, Calif.) and eluted into 100 μl of water.

To obtain the coding region of B19 genome, the primer B19-187FR (CGCTTGTCTTAGTGGCACGTCAAC) (SEQ ID NO:16) was designed from the hairpin region of the virus using sequences available in GenBank (19-HV; AF162273) (SEQ ID NO:17). High fidelity long PCR amplification was performed using the single primer B19-187FR with the HF-2 polymerase kit (BD Biosciences, Palo Alto, Calif.) with 25 cycles of amplification (94° C., 15 s; 55° C., 30 s; 72° C. 4 min; followed by 72° C. extension for 7 min). The amplicon was cloned by blunt ligation into a pCR-Blunt II TOPO® (Invitrogen, San Diego, Calif.) and transformed into One Shot® Top 10 competent E. coli cells (Invitrogen, San Diego, Calif.).

Colonies were screened by hybridization with a ³²P-random-primed B19 probe obtained from pYT103 as previously described for dot blot hybridization (Nguyen et al., 2002), and positive clones were confirmed by sequencing the plasmids using BigDye® terminator cycle sequencing (ABI-Perkin Elmer, Foster City, Calif.). The full-length sequences of both strands were obtained by primer walking.

To obtain the complete hairpin sequence, primers (Table 1) were designed from the cloned sequence and from B19 sequences available in GenBank. PCR amplification was performed using ExTaq polymerase with 30 cycles of amplification. The PCR products were ligated into PCR2.1 TOPO® by TA Cloning® (Invitrogen-Life Technologies), Top10 cells transformed, and the products sequenced as above.

All DNA sequences, and the amino acid sequence of open reading frames, were analyzed using Lasergene® software (DNAStar, Inc., Madison, Wis.). DNA pairwise homology was determined by Lipman-Pearson method with a Ktuple of 2, gap penalty of 4, and deletion penalty of 12. Multiple sequence alignments were determined using the MegAlign program, using the Clustal method with a gap penalty of 10 and gap length penalty of 10.

TABLE 2 List of primer pairs used for PCR SEQ Nucleotide Product ID No. Primer Sequence (5′-3′) (bp) 18 B19-1F CCACGATGCAGCTACAACTT 19 B19-186R GTGAGCGCGCCGCTTGTCTTAGTG 186 20 B19-181F GTGAGCGCGCCGCTTGATCTTAGT 21 B19-1372R AACTTCCACTGTGACTACTG 1195 22 B19-181F GTGAGCGCGCCGCTTGATCTTAGT 23 B19-4899F AACACCACAGGCATGGATAC 518 Discussion

The complete B19 coding region, including half of each ITR, was amplified using PCR. Although several plasmids containing the B19 genome were obtained, only one clone, obtained using the primer B19-187FR, did not contain deletions. This plasmid, designated as pB19-N8 (FIG. 1), was sequenced, and contained a 4844-nucleotide sequence including the entire coding region, and 177 nucleotides of the ITR. The nucleotide sequence of this B19 isolate (J35) had 99.1% identity to that of B19-Au isolate (GenBank M13178) (SEQ ID No:24). The putative NS, VP1 and VP2 capsid proteins had 99.4%, 99.4% and 99.6% homology respectively, at the amino acid level compared to the B19-Au isolate.

The 135 isolate of B19 has a genome of 5592 nucleotides, possessing ITRs of 381 nucleotides in length. The distal 361 nucleotides of these repeats were imperfect palindromes that form double-stranded hairpins. This normally exist in two sequence orientations, “flip” or its reverse-complement “flop”, believed to result from hairpin transfer during replication (Deiss et al., 1990).

The complete sequence analysis of the viral genome (J35) indicates that both the 5′ and 3′ ITRs have two sequence configurations (SEQ ID NO:1 and SEQ ID NO:2) analogous to the flip and flop formats previously reported by Deiss et al. (1990) (FIGS. 2A and 2B; SEQ ID NO:3 and SEQ ID NO:4). Although several base changes within the ITRs were identified compared to the previous published sequence of B19 (Deiss et al., 1990), the size and the positions of the bubbles formed by unpaired nucleotides in these palindromic sequences are conserved among different B19 isolates, suggesting an important role of these structures in the life cycle of B19 virus. In comparison to the previously reported B19 sequence, the hairpin of B19-J35 isolate was shorter by two nucleotides at both 5′ and 3′ ends, but this deletion does not appear to affect viral replication and infection. Unlike other parvoviruses, the hairpins of B19 do not appear to form a Y- or T-shape structure at the turnaround.

Example 2 Construction of B19 Clones

Introduction

There has only been one previous report of the intact ITRs of the human pathogenic parvovirus B19 (Deiss et al., 1990). In Deiss et al., the genome was cloned in two halves, and the sequence of the ITRs obtained. However, Deiss et al. were not unable to successfully ligate the two halves of the genome together nor could they confirm that the ITRs were correct by functional studies. Other attempts to produce an infectious clone were also unsuccessful due to deletions in the ITR sequences (Shade et al., 1986) and the instability of the ITRs in bacterial cells. Our attempts to construct a full-length clone by ligating the ITR sequences to pB19-N8 were repeatedly unsuccessful.

In the present examples, we successfully clone the full-length B19 genome using low incubation temperatures and Sure®2 competent E. coli cells (Strategene, La Jolla, Calif.) that are deficient in major recombination genes. B19 packages equal numbers of both positive and negative DNA strands (Summers et al., 1983) and has a unique BamHI restriction enzyme site in the genome (Cotmore & Tattersall, 1984). These properties were used to clone the full-length B19 genome in two halves (FIG. 3). We also tested whether the full-length B19 genome, especially the ITR sequences, would be stable in the plasmid backbone during the multiple steps of molecular cloning experiments.

Methods

B19 DNA was purified from 50 μl of viremic serum (J35) using the High Pure® Viral Nucleic Acid Kit (Roche, Indianapolis, Ind.) to obtain approximately 1.5 μg of double stranded B19 DNA. Double stranded viral DNA (0.5 μg) was digested with BamHI and both resulting fragments were ligated into BamHI-StuI digested pProEX HTb vector (Invitrogen-Life Technologies). The ligated products were electroporated into electrocompetent Sure®2 E. coli cells (Stratagene) using a BTX electroporator, then the bacteria were immediately plated and incubated overnight at 30° C. The resultant colonies were screened for inserts. To reduce the vector size and eliminate undesired restriction sites, clones that contained the right end of the genome were digested with EcoRV and religated (pB19-42d6). The insert of the plasmid, together with the insert of the left end containing plasmid (pB19-44) was completely sequenced. To create full-length clones, pB19-44 was digested with BamHI and Ecl136II and the fragments containing the left end of the genome were cloned into the BamHI/EheI site of the pB19-42d6 plasmid resulting in pB19-4244 (FIG. 4).

To test the stability of the plasmid containing full-length B19 genome, pB19-4244 was digested with BamHI and religated, and then transformed into Sure®2 cells. After incubation at 30° C. overnight, 18 colonies were picked up from the plates and the bacteria were propagated at 30° C. The plasmids were purified and mapped by restriction digestion with HindIII, BssHII, and SalI. The fragments were then analyzed by agarose-electrophoresis.

Discussion

Of the 192 clones that were analyzed, 5 contained the untruncated 5′ end of the genome, and 2 clones contained the untruncated 3′ end of the genome. All the untruncated clones had the same “flip” format for their ITRs (FIGS. 2A and 2B).

After ligation of the plasmids together, two identical clones consisting of the full-length B19 genome were selected and designated as B19-36 and pB19-4244; GenBank AY386330; SEQ ID NO: 25 (FIG. 4). These full-length clones were sequenced and the sequences of inserts showed 100% identity to the corresponding region in the pB19-N8. The full viral genome was 5592 nucleotides long, with terminal repeat sequences of 381 nucleotides that formed an imperfect palindrome. In comparison to the previously published sequences (Deiss et al., 1990), and the one unpublished sequence in GenBank (B19-HV; AF162273) (SEQ ID NO:17), there were two less nucleotides at the start and end of the genome, resulting in a palindromic sequence of 361. As showed in FIG. 2B, the nucleotide sequences of the flip and flop are slightly different from that reported by Deiss et al. (1990) but the numbers and positions of the unpaired nucleotides in these palindromic sequences are conserved among the two different B19 isolates.

We tested whether the full-length B19 genome, especially the ITR sequences, were able to be stabilized in the plasmid backbone during the multiple steps of molecular cloning experiments. The plasmid pB19-4244 was digested with BamHI and religated, and then transformed into Sure®2 cells. After incubation at 30° C. overnight, 18 colonies were picked up from the plate for purification and mapping by restriction digestion. All of the plasmids tested (18/18) had the correct restriction sites, and there were no deletions in the hairpin sequences. The plasmids were serially passed and then sequenced to confirm the absence of deletions in the hairpin sequences. We found no evidence of deletions under the conditions used in the present study.

Example 3 Introduction of Mutations into a B19 Infectious Clone

Introduction

As an experimental control, a second infectious clone was produced. This clone was generated to have the same nucleotide sequence as plasmid pB19-4244, except for a single nucleotide substitution to confirm that the infectious clone could generate infectious virus. The production of an infectious clone and the ability to manipulate the plasmid will allow the genome to be studied more systematically.

Method

A second infectious clone was produced by site directed mutagenesis. The cytosine at position 2285 (B19-J35 isolate) was replaced with a thymine to generate a recognition site for restriction enzyme DdeI to produce a naturally existing variant of B19 (B19-Wi isolate, GenBank M24682; SEQ ID NO:26). The full-length plasmid pB19-4244 was cut with NheI and the 5′ overhang filled in using T4 polymerase. The linearized plasmid was redigested with XbaI, the B19 fragment (from nucleotide 1247 to 3423 in the genome of B19-J35 isolate) ligated into an XbaI-Ecl136II-digested pBluescriptII® KS+ phagemid vector (Stratagene), and site-specific mutagenesis (C2285T) was performed using the QuikChange® Site-directed Mutagenesis Kit (Stratagene) and primers CMCf (CATTTGTCGGAAGCTCAGTTTCCTCCGAAG; SEQ ID NO:27) and CMCr (CTTCGGAGGAAACTGAGCTTCCGACAAATG; SEQ ID NO:28). To eliminate an undesired XbaI restriction site in the vector sequence of the plasmid pB19-4244, the plasmid was digested with Ecl136 II-XhoI enzymes, the XhoI overhang was blunted with T4 polymerase, and the plasmid was religated (plasmid pB19-4244d, FIG. 5). The plasmid with the B19 fragment containing C2285T mutation was digested with MscI-XbaI and the B19 fragment was ligated into the MscI-XbaI digested pB19-4244d plasmid resulting the pB19-M20 clone (FIG. 6).

Discussion

Although the cloned sequence was 99% identical to the B19-Au sequence, it was observed that there was a single nucleotide difference between the J35 sequence (and B19-Au) and the published sequence of another isolate B19-Wi (GenBank M24682) that would convert a BsrI site in J35 to a DdeI site. This site was within the RT-PCR product amplified with the primer pair of B19-2255 and B19-2543 and could potentially be used to distinguish transcripts, and hence, viruses with the different sequence. We therefore constructed a second plasmid, pB19-M20 (FIG. 6), that contained the identical full-length clone, but in which the cytosine at the nucleotide of 2285 was replaced by thymine (C2285T).

Example 4 Infection of Cells with B19 and Detection of Replicative Forms of B19 in Infected Cells

Introduction

During the replication of parvovirus B19, the viral single-stranded DNA is converted to a double-stranded replicative form which has either an “extended” or a “turnaround” form at the terminal regions. These intermediate structures provide evidence for viral DNA replication and can be distinguished by BamHI restriction enzyme digestion (Cotmore & Tattersall, 1984) (FIG. 7).

To test whether the B19 genome inserted in pB19-4244 could be excised from the flanking vector sequences and produce progeny viral DNA, we compared Southern blot analysis of the DNA purified from the cells transfected with either plasmids cut with SalI enzyme (which releases the full-length B19 genome from the plasmid, FIG. 4) or intact plasmids. Additionally, RT-PCR was used to detect transcripts for viral capsids in RNA recovered from transfected cells. The presence or absence of B19 capsid proteins was detected via immunofluorescent microscopy. By these experimental methods, the presence, transcription, and expression of the capsid gene could be confirmed.

Method

The conditions and reagents for transfecting plasmid DNA into UT7/Epo-S1 cells were first optimized using the plasmid pEGFP-F (BD Biosciences, Palo Alto, Calif.) that encodes farnesylated enhanced green fluorescent protein (EGFP). Cells were examined at daily intervals for expression of EGFP by UV microscopy and by FACS analysis. Conditions that gave the maximum number of cells expressing EGFP with minimum cytotoxicity were chosen. For subsequent experiments UT7/Epo-S1 cells were transfected using the AMAXA® Cell Line Nucleofector™ kit R according to manufacture's instruction (AMAXA Biosystems Inc., Nattermannallee, Germany). The cells were harvested at various times posttransfection and used for DNA, RNA, and immunofluorescence studies. For infection studies, cells were harvested at 72 h posttransfection, washed free of inoculums using fresh culture medium, and cell lysate prepared by three cycles of freeze/thawing. After centrifugation at 10,000 g for 10 min, the clarified supernatant was treated with RNase (final concentration of 1 U/μl, Roche Applied Science, Indianapolis, Ind.) and collected for further infections.

Total RNA was extracted from the UT7/Epo-S1 cells (2×10⁵) using RNA STAT60™ (Tel-Test Inc., Friendswood, Tex.). Residual DNA was removed by DNAse I treatment (final concentration, 90 U/ml) for 15 min at room temperature. RNA was converted to cDNA with random hexamers and SuperScript™ II (Invitrogen), and RT-PCR for the spliced capsid transcripts was performed with primers B19-1 (5′GTTTTTTGTGAGCTAACTA3′; SEQ ID NO:6) and B19-9 (5′CCACGATGCAAGCTACAACTT3; SEQ ID NO:7) as described in (Nguyen et al., 2002).

To exclude the possibility that the transcripts detected were derived from laboratory contamination of B19 viral RNA, the cDNA derived from the pM20-transfected cells were PCR amplified by using a primer pair of B19-2255 (GGAACCAGTTCAGGAGAATCA; SEQ ID NO:8) and B19-2543 (TGGCAGCTACATCGCACCAA; SEQ ID NO:9), which annealed proximal to the region containing the site of mutagenesis (C2285T). After purification using QIAquick® PCR Purification Kit (Qiagen Inc., Valencia, Calif.), the PCR products were digested with DdeI at 37° C. for 2 h.

Immunofluorescence. Infected or transfected cells were harvested and cytocentrifuged (1500 rpm for 8 mins in a Shandon cytospin 2 cytocentrifuge). The cells were fixed in acetone:methanol (1:1) at −20° C. for 5 min, washed twice in phosphate buffered saline (PBS) containing 0.1% fetal bovine serum, and incubated with a murine anti-B19 capsid protein monoclonal antibody (521-5D, gift of Larry Anderson, CDC) in PBS with 10% fetal calf serum for 1 hr at 37° C. After washing the slides twice in PBS, the slides were incubated with fluorescein isothiocyanate (FITC)-labeled goat anti-mouse IgG antibody (Jackson ImmunoResearch Laboratories, Inc., West Grove, Pa.) in PBS with 10% fetal calf serum and counterstained with Evans Blue for 30 mins at 37° C., washed in PBS, and examined by UV microscopy.

Southern blot analysis of B19 DNA. DNA was extracted from B19 infected UT7/Epo-S1 cells (5×10⁵) as previously described (Shimomura et al., 1992). Briefly, 5×10⁵ cells were incubated with 100 mM NaCl, 10 mMTris-HCl (pH 7.5), 0.5% sodium dodecylsufate (SDS), 5 mM EDTA, and 200 μg/ml proteinase K overnight at 37° C. followed by phenol-chloroform extraction. For some experiments high and low-molecular weight DNA were separated by the Hirt method (Hirt, 1967). Purified DNA (400 ng) was digested with 20 U of BamH I (single cut in B19) or EcoRI (no cut in B19) at 37° C. for 4 h. The fragments were then separated by agarose-electrophoresis, transferred to a nylon membrane (Nylon+, Amersham), and hybridized with a ³²P-random-primed probe of the complete B19 coding region as previously described (Shimomura et al., 1992).

Discussion

The plasmid pEGFP-F was used to optimize the conditions for transfecting UT7/Epo-S1 cells. Although standard electroporation and liposomes were also tried, the best results were obtained using the AMAXA® Cell Line Nucleofector System™. The highest transfection efficiency (˜70%) with minimum cytotoxicity (˜20%) was achieved with reagent R and T-20 program using 3 μg pEGFP DNA and 2×10⁶UT7/Epo-S1 cells, following the manufacturer's instructions (AMAXA Biosystems Inc., Cologne, Germany).

UT7/Epo-S1 cells were transfected with plasmids pB19-4244, pB19-M20, and pB19-N8 under the same conditions, and harvested at 72 h post-transfection. The RT-PCR and immunofluorescence assay were performed to detect the viral spliced transcripts and capsid proteins. After RT-PCR, two amplicons of 253 bp and 133 bp, representing the alternative spliced transcripts of B19 capsid gene, were detected in the cells transfected with either plasmid (FIG. 8). By immunofluorescence assay, B19 capsid protein was also detected in the transfected cells, with approximately 15% of the cells having a positive signal when transfected with pB19-4244 and (FIG. 9B) and 5% with pB19-pN8 (FIG. 9C). There was a significant difference in the number of positive cells between the two different plasmid constructs although the same amount of plasmid DNA was introduced into the cells under identical conditions. Infection with B19 wild-type virus (J35 isolate) gave approximately 20% positive cells (FIG. 9A).

At 72 h posttransfection, the DNA was extracted from the cells and incubated with the restriction endonuclease EcoRI (no cuts in the parvovirus B19 genome) or BamHI (a single cut in the parvovirus genome). As in B19 infection of UT7/Epo-S1 cells, distinct doublets of 1.5 kb and 1.4 kb were detected in all the transfected cell samples digested with BamHI, but not in the plasmid controls (FIGS. 10 and 11). Although a portion of the signal for the 4.1 and 1.5 kb bands in FIG. 10 is contributed by the transfected DNA, the 1.4 kb band is a definitive marker for viral genome replication. In addition, a band with a molecular size of 5.6 kb, which corresponds to the size of the viral B19 genome, was detected in EcoRI-digested DNA from the cells transfected with undigested (SalI) plasmid pB19-M20 (FIG. 11). This indicated that viral progeny DNA was produced because neither the B19 genome nor vector contain an EcoRI restriction enzyme site. Although equal amounts of DNA of either SalI-digested plasmid or whole plasmid were introduced into the cells, the band density of the replication intermediates in the sample of SalI-digested fragment appeared to be stronger. This suggested that the replication process was facilitated when the viral genome was released from the vector backbone.

Example 5 Confirmation of B19 Infectious Virus

Introduction

To determine if infectious virus were generated from the UT7/Epo-S1 cells transfected with plasmid pB19-4244 or pB19-M20, the supernatant from the cell lysates was tested for the detection of spliced transcripts of viral capsid genes by RT-PCR. We also performed in vitro neutralization assays to confirm that the infectivity of the cell lysates was mediated by newly synthesized B19 virons. Finally to confirm that the viral transcripts in the inoculated cells were being generated from the infectious clone and not from laboratory contamination of wild type J35 virus, we also used the second infectious clone (pB19-M20) that carried a DdeI site that was present in other B19 isolates but not in J35 virus.

Method

For infection studies, 2×10⁴ of UT7/Epo-S1 cells in 10 μl IMDM were mixed with an equal volume of sample or positive control (J35 serum diluted to contain 10⁸ B19 genome copies) and incubated at 4° C. for 2 h to allow for maximum virus-cell interaction. The cells were then diluted to 2×10⁵ cells/ml in the culture medium, and incubated at 37° C., in 5% CO₂. Cells were harvested at 3 days post infection and tested for evidence of infection by detection of viral transcripts and protein expression. To determine if infectious virus were generated from the UT7/Epo-S1 cells transfected with plasmid pB19-4244 or pB19-M20, the supernatant from the cell lysates was tested for the detection of spliced transcripts of viral capsid genes by RT-PCR. Plasmid pB19-N8, which does not contain intact ITRs and should not produce infectious virus, was used as a negative control. B19 infected UT7/Epo-S1 cells were used as a positive control.

In vitro neutralization assays were performed to test whether neutralizing monoclonal antibodies against parvovirus B19 capsids were able to block the infection caused by the cell lysates of transfected cells. The clarified cell lysates prepared from the transfected cells were mixed with monoclonal antibody A and E (Yoshimoto et al., 1991) at a dilution of 1:10, and incubated at room temperature for 2 h. The anti-B19 monoclonal antibody A without neutralizing activities was used as control. The infection studies were performed as described above.

Discussion

As observed previously, following transfection, spliced transcripts were detected in all the samples including cells transfected with pB19-N8 (FIG. 8). Immediately after inoculation of the clarified supernatant into the UT7/Epo-S1 cells, no RT-PCR product was detected in any of the sample (FIG. 12A), indicating that there was no carry-over of the RNA from the transfected cells. At 72 h post-inoculation spliced transcripts were detected in the samples derived from the cells transfected with pB19-4244 and pB19-M20, but not with pB19-N8 (FIG. 12B), confirming that the full-length viral genome containing complete ITRs is essential for generation of infectious viral particles. In addition, no viral transcripts were detected in cells in which the plasmids were directly incubated with the cells (no electroporation) (FIG. 12B), suggesting that the detection of transcripts in the cells inoculated with transfected-cell lysate was due to the production of infectious B19 virus from the plasmid.

The infected cultures were also examined for the production of parvovirus B19 capsid proteins. At 72 h post-inoculation capsid proteins could be detected in the nuclei and cytoplasm of cells with the supernatants derived from either B19 infection or pB19-M20 transfection (FIGS. 13A and 13B), but not in the cells inoculated with either pB19-N8 cell lysate (FIG. 13C), or directly with plasmid.

We also performed in vitro neutralization assays to confirm that the infectivity of the cell lysates was mediated by newly synthesized B19 virons. Incubation of the cell lysates with neutralizing monoclonal antibody E (Yoshimoto et al., 1991) reduced the infectivity to undetectable levels in the WA testing. In contrast, incubation with a similar concentration of monoclonal antibody known to be non-neutralizing (monoclonal antibody A) had no effect on infection. This result further supports our infection experiment, indicating that infectious viral particles were produced from the cells transfected with the plasmids containing full-length B19 genome.

Finally to confirm that the viral transcripts in the inoculated cells were being generated from the infectious clone and not from laboratory contamination of wild type J35 virus, we constructed the second infectious clone (pB19-M20) that carried a DdeI site that was present in other B19 isolates but not in J35 virus. The sequencing analysis of the plasmids constructed in site-specific mutagenesis showed that full-length B19 genome including complete ITR was stable during serial passages in Sure2 bacteria cells, demonstrating the capacity for manipulating and stably passaging the infectious clone. After transfection, the viral transcripts were tested by restriction enzyme digestion for the presence of the artificially generated DdeI site (FIGS. 14 and 15). No DdeI site was present in transcripts generated by wild-type B19-J35 isolate infection. A DdeI was present only in transcripts from cells infected with lysate from pB19-M20-transfected cells (FIG. 15).

Example 6 Identification of Viral Proteins Involved in B19 Infection

Introduction

In common with other parvoviruses, B19 has a small (22 nm), nonenveloped, icosahedral capsid packaging a single-stranded DNA. The B19 genome has approximately 5,600 nucleotides. The ends of the genome are long inverted terminal repeats (ITR) of 383 nucleotides in length, of which the distal 365 nucleotides form an imperfect palindrome (Deiss et al., 1990). Transcription of the B19 viral genome is controlled by a single promoter p6 that regulates synthesis of nine viral transcripts to produce one nonstructural protein (NS), two capsid proteins (VP1 and VP2), and two small proteins (11-kDa and 7.5-kDa) of unknown function (St. Amand et al., 1993, Virology, 195:448-455). Additionally, there is a putative open reading frame encoding a functionally unknown small protein X (9-kDA).

In order to experimentally define the role of these genes, we utilized the infectious B19 clone described in Example 1 to generate knockout mutants in which the translational start codon for each of the described viral genes was substituted with a stop codon.

Methods

To knockout expression of VP1, 7.5-kDa protein, or protein X, the translational initiation site (ATG) at 5′ of the gene was replaced with a stop codon (TAG). Plasmid pB19-M20/VP1(−) contained a knockout mutation for VP1. Plasmid pB19-M20/7.5(−) contained a knockout mutation for 7.5 kDa protein. Plasmid pB19-M20/X(−) contained a knockout mutation for protein X.

To prepare these knockout plasmids, the full-length plasmid pB19-4244 was cut with NheI and the 5′ overhang filled in using T4 polymerase. The linearized plasmid was redigested with XbaI, the B19 fragment (from nucleotide 1249 to 3425 in the genome of B19-J35 isolate) ligated into an XbaI-Ecl13611-digested pBluescriptII KS+ cloning vector (Stratagene), and site-specific mutagenesis was performed using the Quickchange Site-directed Mutagenesis Kit (Stratagene). The primers shown in Table 3 were used in the site-specific mutagenesis.

TABLE 3 Knockout PCR primers SEQ ID Gene Primer Nucleotide Sequence NO Mutation VP1 Forward 5′GCAAAGCTTTGTAGATTTAG SEQ ID A2624T and AGTAAAGAAAGTGGCAAATGGT NO: 29 T2625A GGG3′ Reverse 5′CCCACCATTTGCCACTTTCT SEQ ID TTACTCTAAATCTACAAAGCTT NO: 30 TGC3′ 7.5-kDa Forward 5′GATTTCCCTGGAATTATAGC SEQ ID A2084T and protein AGATGCCCTCCACCCAGACC3′ NO: 31 T2083A Reverse 5′GGTCTGGGTGGAGGGCATCT SEQ ID GCTATAATTCCAGGGAAATC3′NO: 32 Protein X Forward 5′AGTCATCATTTTCAAAGTCT SEQ ID A2874T and AGGACAGTTATCTGACCACC3′ NO: 33 T2875A Reverse 5′GGTGGTCAGATAACTGTCCT SEQ ID AGACTTTGAAAATGATGACT3′ NO: 34

To eliminate an undesired XbaI restriction site in the vector sequence of the plasmid pB19-4244, the plasmid was digested with Ecl13611-XhoI enzymes, the XhoI overhang was blunted with T4 polymerase, and the plasmid was religated (plasmid pB19-4244d).

To knockout expression of 11-kDa protein, the third translational initiation site (ATG) at 5′ of the 11-kDa protein gene was replaced with a stop codon (TAG). Plasmid pB19-M20/11(−) contained a knockout mutation for 11-kDa protein.

The full-length plasmid p1319-4244 described in Example 1 was cut with XabI and BbvClI and the B19 fragment (from nucleotide 1247 to 3423 in the genome of B19-J35 isolate) was ligated into an XbaI-BbvCI-digested pBluescriptII KS+cloning vector (Stratagene), and site-specific mutagenesis (A4917T, T4918A) was performed using the Quickchange Site-directed Mutagenesis Kit (Stratagene) and primers of P11(−)F3 (5′CACCACAGACATGGATTAGAAAAGCCTGAAGAATTGTGGAC3′; SEQ ID NO:35), and P11(−)R3 (5′GTCCACAATTCTTCAGGCTTTTCTAATCCATGTCTGTGGTG3′; SEQ ID NO:36). Plasmid with the B19 fragment containing both the A4917T and T4918A mutations was digested with XbaI-BbvCI and the fragment was ligated into XbaI-BbvCI aI digested pB19-4244d plasmid.

To disrupt the expression of NS protein, the full-length plasmid pB19-4244 was cut with AflII (at nucleotide 756 in B19 genome) and the 5′ overhang filled in using T4 polymerase. The linearized plasmid was religated with T4 ligase, which generated a stop codon and disrupted the open reading frame of NS. The plasmid was named pB19-M20/NS(−).

To obtain the ITR deletion mutant, the primer B19-187FR (Table 1) was designed from the hairpin region of the virus using sequences available in GenBank (19-HV; Genbank accession number AF162273). High fidelity long PCR amplification was performed using the single primer B19-187FR with a HF-2 polymerase kit (BD Biosciences, Palo Alto, Calif.) with 25 cycles of amplification (94° C. for 15 sec; 55° C. for 30 sec; 72° C. for 4 min; followed by extension at 72° C. for 7 min). The amplicon was cloned by blunt ligation into a pCR-Blunt II TOPO (Invitrogen-Life Technologies, San Diego, Calif.) and transformed into Top10 cells (Invitrogen-Life Technologies).

Colonies were screened by hybridization with a ³²P-random-primed B19 probe obtained from pYT103 as previously described for dotblot hybridization, (Nguyen et al., 2002) and positive clones confirmed by sequencing the plasmids using BigDye terminator cycle sequencing (ABI-Perkin Elmer, Foster City, Calif.). The full-length sequences of both strands were obtained by primer walking. One clone (pB19-N8) contained a 4844-nucleotide sequence including the entire coding region, and 177 nucleotides of the ITR at both 5′ and 3′ ends (GenBank AY386330).

UT7/Epo-S1 cells were transfected with the B19 variant plasmids using the AMAXA Cell Line Nucleofector™ kit R according to the manufacture's instructions (AMAXA Biosystems Inc., Cologne, Germany). The cells were harvested at various times post-transfection and used for DNA, RNA, and immunofluorescence studies. For infection studies, cells were harvested 72 h post-transfection, washed free of inoculums using fresh culture medium, and cell lysates prepared by three cycles of freeze/thawing. After centrifugation at 10,000 g for 10 min, the clarified supernatant was treated with RNase (final concentration of 1 U/μl, Roche) and collected for further infections.

B19 variant transcripts were detected using RT-PCR. Total RNA was extracted from the UT7/Epo-S1 cells (2×10⁵) using RNA STAT60 (Tel-Test Inc., Friendswood, Tex.). Residual DNA was removed by DNAse I treatment (final concentration, 90 U/ml) for 15 min at room temperature. RNA was converted to cDNA with random hexamers and SuperScript II and RT-PCR for the spliced capsid transcripts was performed with primers B19-1 and B19-9 as described in Example 4.

To exclude the possibility the detected transcripts detected were derived from laboratory contamination of B19 viral RNA, cDNA derived from pM20-transfected cells were PCR amplified by using a primer pair of B19-2255 and B19-2543 (Table 1), which targeted on the region containing the site of mutagenesis (C2285T). After purified by using QIAquick PCR Purification Kit (Qiagen Inc., Valencia, Calif.), the PCR products were digested with DdeI at 37° C. for 2 h.

B19 variants were analyzed for capsid protein expression using the indirect fluorescent antibody assay described in Example 4. Infected or transfected cells were harvested and cytocentrifuged (1500 rpm for 8 mins in a Shandon cytospin 2 cytocentrifge). The cells were fixed in acetone:methanol (1:1) at −20° C. for 5 min and washed twice in phosphate buffered saline (PBS) containing 0.1% fetal bovine serum, and incubated with a mouse monoclonal antibody specific to B19 capsid proteins (521-5D, obtained from Dr. Larry Anderson, CDC) or a rabbit polyclonal antibody to 11-kDa protein in PBS with 10% fetal calf serum for 1 hr at 37° C.

For double IFA staining, a lissamine rhodamine-labeled goat anti-mouse IgG and fluorescein isothiocyanate-labeled goat anti-rabbit IgG (Jackson ImmunoResearch Laboratories, Inc., West Grove, Pa.) were used as secondary antibodies. The B19 variants were then examined for capsid proteins using confocal microscopy (LSM 510, Leica).

Discussion

No infectious B19 virus was detected in cells transfected with NS, VP1, or 11-kDa protein knockout plasmids. As shown in FIGS. 16A-E, immediately following transfection, spliced transcripts were detected in cells transfected with pB19-M20 (FIG. 16A), pB19-M20/VP1(−) (FIG. 16B), pB19-M20/11(−) (FIG. 16C), pB19-M20/7.5(−) (FIG. 16D), pB19-M20/X (−) (FIG. 16E), or pB19-N8 (ITR deletion; FIG. 16F). No spliced transcripts were detected in cells transfected with pB19-M20/NS(−) immediately following transfection (FIG. 16A).

Immediately following infection of UT7/Epo-S1 cells with clarified supernatant from the transfected cells, no RT-PCR product was detected in any of the cells, indicating that there was no carry-over of the RNA from the transfected cells (FIGS. 16A-F). Seventy-two h post-inoculation, spliced transcripts were detected in cells infected with supernatant derived from cells transfected with pB19-M20 (FIG. 16A), pB19-M20/7.5(−) (FIG. 16E), or pB19-M20/X (−) (FIG. 16E), but not pB19-M20/NS(−) (FIG. 16A), pB19-M20/VP1(−) (FIG. 16B), pB19-M20/11(−) (FIG. 16C), or pB19-N8 (FIG. 16F). The data in FIG. 16 indicated that knocking out expression of 11-kDa protein, VP1, NS, or ITR reduced the production of infectious viral particles to an undetectable level.

Knocking out 11-kDa protein changed the expression and distribution pattern of B19 viral capsid protein (FIGS. 17A-D). In cells transfected with wild-type infectious clone, viral capsid protein first appeared in the host cell nucleus and was transported into the cytoplasm at a late stage of infection. Capsid protein was either evenly distributed or formed fine clusters in the cytoplasm and nucleus. In cells transfected with 11-kDa protein knockout plasmids, production of viral capsid protein was significantly decreased. Viral capsid protein formed rough clusters in the nucleus and could not be transported to cytoplasm (FIGS. 17 C and 17D), suggesting 11-kDa protein may be involved in regulation of viral promoter activity or viral capsid transportation.

Taken together, the data in FIGS. 16A-F and FIGS. 17A-D indicated that 11-kDa protein may play an important role in replication of B19 and confirmed that 11-kDa protein, in addition to ITR sequences and VP2, NS, and VP I proteins, is essential for production of infectious particles of B19 parvovirus. 

We claim:
 1. A method for cloning a parvovirus B19 viral genome comprising: (a) introducing a vector comprising all or a portion of a parvovirus B19 genome into a prokaryotic cell that is deficient in at least one recombinase enzyme, wherein the method uses a full length inverted terminal repeat (ITR) at the 5′ end and the 3′ end of the genome, wherein the ITR located at the 5′ end comprises SEQ ID NO:1 or SEQ ID NO:2, and wherein the ITR located at the 3′ end comprises SEQ ID NO:1 or SEQ ID NO:2; (b) incubating the cells at about 25° C. to 35° C.; and (c) recovering the vector from the prokaryotic cells.
 2. The method of claim 1, wherein the ITR located at the 5′ end or the 3′ end comprises SEQ ID NO:1.
 3. The method of claim 1, wherein the ITR located at the 5′ end or the 3′ end comprises SEQ ID NO:2.
 4. The method of claim 1, wherein the viral genome further comprises a nucleic acid sequence encoding at least one or all of a parvovirus B19 VP1, a parvovirus B19 VP2, a parvovirus B19 nonstructural protein, or a parvovirus B19 11-kpa protein.
 5. The method of claim 1, wherein the viral genome is a full length parvovirus B19 genome.
 6. An isolated cell comprising an infectious clone of parvovirus comprising the parvovirus B19 ITR sequence set forth in SEQ ID NO: 1 and/or SEQ ID NO:
 2. 7. A method for producing an infectious virus of parvovirus B19, comprising: introducing a vector comprising an infectious clone of parvovirus B19 into a population of cells, wherein the infectious clone of parvovirus B19 comprises a parvovirus B19 genome, wherein the genome comprises a full length inverted terminal repeat (ITR) at the 5′ end and the 3′ end, wherein the ITR located at the 5′ end comprises SEQ ID NO:1 or SEQ ID NO:2, wherein the ITR located at the 3′ end comprises SEQ ID NO:1 or SEQ ID NO:2; and wherein the vector is present in at least about 15% of the cells; and incubating the cells under conditions to allow for viral replication.
 8. The method of claim 7, wherein the cells are eukaryotic cells.
 9. The method of claim 7, wherein introducing the vector into the population of cells is conducted by electric current.
 10. The method of claim 9, wherein the cells are exposed to an electrical pulse comprising a field strength of about 2kV/cm to about 10kV/cm, a duration of at least about usec, and a current of at least about 1 A followed by a current flow of about 1 A to about 3 A for at least 10 msec. 